Atomic theory

In chemistry and physics, atomic theory is a scientific theory of the nature of matter, which states that matter is composed of discrete units called atoms. It began as a philosophical concept in ancient Greece and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of atoms.

The word atom comes from the Ancient Greek adjective atomos, meaning "indivisible".[1] 19th century chemists began using the term in connection with the growing number of irreducible chemical elements. Around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "uncuttable atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments, such as neutron stars, extreme temperature and pressure prevents atoms from existing at all.

Since atoms were found to be divisible, physicists later invented the term "elementary particles" to describe the "uncuttable", though not indestructible, parts of an atom. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.

Helium atom QM
The current theoretical model of the atom involves a dense nucleus surrounded by a probabilistic "cloud" of electrons


Philosophical atomism

The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India. The word "atom" (Greek: ἄτομος; atomos), meaning "uncuttable", was coined by the Pre-Socratic Greek philosophers Leucippus and his pupil Democritus (c. 460 – c. 370 BC).[2][3][4][5] Democritus taught that atoms were infinite in number, uncreated, and eternal, and that the qualities of an object result from the kind of atoms that compose it.[3][4][5] Democritus's atomism was refined and elaborated by the later Greek philosopher Epicurus (341 – 270 BC), and by the Roman Epicurean poet Lucretius (c. 99 – c. 55 BC).[4][5] During the Early Middle Ages, atomism was mostly forgotten in western Europe, but survived among some groups of Islamic philosophers.[4] During the 12th century, atomism became known again in western Europe through references to it in the newly-rediscovered writings of Aristotle.[4]

In the 14th century, the rediscovery of major works describing atomist teachings, including Lucretius's De rerum natura and Diogenes Laërtius's Lives and Opinions of Eminent Philosophers, led to increased scholarly attention on the subject.[4] Nonetheless, because atomism was associated with the philosophy of Epicureanism, which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable by most European philosophers.[4] The French Catholic priest Pierre Gassendi (1592 – 1655) revived Epicurean atomism with modifications, arguing that atoms were created by God and, though extremely numerous, are not infinite.[4][5] Gassendi's modified theory of atoms was popularized in France by the physician François Bernier (1620 – 1688) and in England by the natural philosopher Walter Charleton (1619 – 1707).[4] The chemist Robert Boyle (1627 – 1691) and the physicist Isaac Newton (1642 – 1727) both defended atomism and, by the end of the 17th century, it had become accepted by portions of the scientific community.[4]

John Dalton

Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. The first was the law of conservation of mass, closely associated with the work of Antoine Lavoisier, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products).[6] The second was the law of definite proportions. First established by the French chemist Joseph Louis Proust in 1799,[7] this law states that if a compound is broken down into its constituent chemical elements, then the masses of the constituents will always have the same proportions by weight, regardless of the quantity or source of the original substance.

John Dalton studied and expanded upon this previous work and defended a new idea, later known as the law of multiple proportions: if the same two elements can be combined to form a number of different compounds, then the ratios of the masses of the two elements in their various compounds will be represented by small whole numbers. For example, Proust had studied tin oxides and found that there is one type of tin oxide that is 88.1% tin and 11.9% oxygen and another type that is 78.7% tin and 21.3% oxygen (these are tin(II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found several examples of such instances of integral multiple combining proportions, and asserted that the pattern was a general one. Most importantly, he noted that an atomic theory of matter could elegantly explain this law, as well as Proust's law of definite proportions. For example, in the case of Proust's tin oxides, one tin atom will combine with either one or two oxygen atoms to form either the first or the second oxide of tin.[8]

Dalton believed atomic theory could explain why water absorbed different gases in different proportions - for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen.[9] Dalton hypothesized this was due to the differences in mass and complexity of the gases' respective particles. Indeed, carbon dioxide molecules (CO2) are heavier and larger than nitrogen molecules (N2).

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion.

Daltons symbols
Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).

In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss there exactly how he obtained these figures.[9] The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of Thomson's textbook, A System of Chemistry. Finally, Dalton published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and 1810.

Dalton estimated the atomic weights according to the mass ratios in which they combined, with the hydrogen atom taken as unity. However, Dalton did not conceive that with some elements atoms exist in molecules—e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O).[10] This, in addition to the crudity of his equipment, flawed his results. For instance, in 1803 he believed that oxygen atoms were 5.5 times heavier than hydrogen atoms, because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water was HO. Adopting better data, in 1806 he concluded that the atomic weight of oxygen must actually be 7 rather than 5.5, and he retained this weight for the rest of his life. Others at this time had already concluded that the oxygen atom must weigh 8 relative to hydrogen equals 1, if one assumes Dalton's formula for the water molecule (HO), or 16 if one assumes the modern water formula (H2O).[11]


The flaw in Dalton's theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies).[12] Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance: since two liters of hydrogen will react with just one liter of oxygen to produce two liters of water vapor (at constant pressure and temperature), it meant a single oxygen molecule splits in two in order to form two particles of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms.

Brownian Motion

In 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it.[13] This model was validated experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory).

Discovery of subatomic particles

JJ Thomson Crookes Tube Replica
JJ Thomson Cathode Ray 2 explained
The cathode rays (blue) were emitted from the cathode, sharpened to a beam by the slits, then deflected as they passed between the two electrified plates.

Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays.[14]

A Crookes tube is a sealed glass container in which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being a form of light, were composed of very light negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists). He measured the mass-to-charge ratio and discovered it was 1800 times smaller than that of hydrogen, the smallest atom. These corpuscles were a particle unlike any other previously known.

Thomson suggested that atoms were divisible, and that the corpuscles were their building blocks.[15] To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model[16] as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary).

Discovery of the nucleus

Geiger-Marsden experiment expectation and result
The Geiger-Marsden experiment
Left: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.
Right: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.

Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center.

In the Geiger–Marsden experiment, Hans Geiger and Ernest Marsden (colleagues of Rutherford working at his behest) shot alpha particles at thin sheets of metal and measured their deflection through the use of a fluorescent screen.[17] Given the very small mass of the electrons, the high momentum of the alpha particles, and the low concentration of the positive charge of the plum pudding model, the experimenters expected all the alpha particles to pass through the metal foil without significant deflection. To their astonishment, a small fraction of the alpha particles experienced heavy deflection. Rutherford concluded that the positive charge of the atom must be concentrated in a very tiny volume to produce an electric field sufficiently intense to deflect the alpha particles so strongly.

This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge. Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection.[18]

First steps toward a quantum physical model of the atom

The planetary model of the atom had two significant shortcomings. The first is that, unlike planets orbiting a sun, electrons are charged particles. An accelerating electric charge is known to emit electromagnetic waves according to the Larmor formula in classical electromagnetism. An orbiting charge should steadily lose energy and spiral toward the nucleus, colliding with it in a small fraction of a second. The second problem was that the planetary model could not explain the highly peaked emission and absorption spectra of atoms that were observed.

Bohr atom animation 2
The Bohr model of the atom

Quantum theory revolutionized physics at the beginning of the 20th century, when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in discrete amounts known as quanta (singular, quantum). In 1913, Niels Bohr incorporated this idea into his Bohr model of the atom, in which an electron could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy.[19] Under this model an electron could not spiral into the nucleus because it could not lose energy in a continuous manner; instead, it could only make instantaneous "quantum leaps" between the fixed energy levels.[19] When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra).[19]

Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't predict those of multielectron atoms. Worse still, as spectrographic technology improved, additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In 1916, Arnold Sommerfeld added elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.

Discovery of isotopes

While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one element at each position on the periodic table.[20] The term isotope was coined by Margaret Todd as a suitable name for these elements.

That same year, J.J. Thomson conducted an experiment in which he channeled a stream of neon ions through magnetic and electric fields, striking a photographic plate at the other end. He observed two glowing patches on the plate, which suggested two different deflection trajectories. Thomson concluded this was because some of the neon ions had a different mass.[21] The nature of this differing mass would later be explained by the discovery of neutrons in 1932.

Discovery of nuclear particles

In 1917 Rutherford bombarded nitrogen gas with alpha particles and observed hydrogen nuclei being emitted from the gas (Rutherford recognized these, because he had previously obtained them bombarding hydrogen with alpha particles, and observing hydrogen nuclei in the products). Rutherford concluded that the hydrogen nuclei emerged from the nuclei of the nitrogen atoms themselves (in effect, he had split a nitrogen).[22]

From his own work and the work of his students Bohr and Henry Moseley, Rutherford knew that the positive charge of any atom could always be equated to that of an integer number of hydrogen nuclei. This, coupled with the atomic mass of many elements being roughly equivalent to an integer number of hydrogen atoms - then assumed to be the lightest particles - led him to conclude that hydrogen nuclei were singular particles and a basic constituent of all atomic nuclei. He named such particles protons. Further experimentation by Rutherford found that the nuclear mass of most atoms exceeded that of the protons it possessed; he speculated that this surplus mass was composed of previously-unknown neutrally charged particles, which were tentatively dubbed "neutrons".

In 1928, Walter Bothe observed that beryllium emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles. It was later discovered that this radiation could knock hydrogen atoms out of paraffin wax. Initially it was thought to be high-energy gamma radiation, since gamma radiation had a similar effect on electrons in metals, but James Chadwick found that the ionization effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction. In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring the energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton. Chadwick now claimed these particles as Rutherford's neutrons.[23] For his discovery of the neutron, Chadwick received the Nobel Prize in 1935.

Quantum physical models of the atom

The five filled atomic orbitals of a neon atom separated and arranged in order of increasing energy from left to right, with the last three orbitals being equal in energy. Each orbital holds up to two electrons, which most probably exist in the zones represented by the colored bubbles. Each electron is equally present in both orbital zones, shown here by color only to highlight the different wave phase.

In 1924, Louis de Broglie proposed that all moving particles—particularly subatomic particles such as electrons—exhibit a degree of wave-like behavior. Erwin Schrödinger, fascinated by this idea, explored whether or not the movement of an electron in an atom could be better explained as a wave rather than as a particle. Schrödinger's equation, published in 1926,[24] describes an electron as a wavefunction instead of as a point particle. This approach elegantly predicted many of the spectral phenomena that Bohr's model failed to explain. Although this concept was mathematically convenient, it was difficult to visualize, and faced opposition.[25] One of its critics, Max Born, proposed instead that Schrödinger's wavefunction described not the electron but rather all its possible states, and thus could be used to calculate the probability of finding an electron at any given location around the nucleus.[26] This reconciled the two opposing theories of particle versus wave electrons and the idea of wave–particle duality was introduced. This theory stated that the electron may exhibit the properties of both a wave and a particle. For example, it can be refracted like a wave, and has mass like a particle.[27]

A consequence of describing electrons as waveforms is that it is mathematically impossible to simultaneously derive the position and momentum of an electron. This became known as the Heisenberg uncertainty principle after the theoretical physicist Werner Heisenberg, who first described it and published it in 1927.[28] This invalidated Bohr's model, with its neat, clearly defined circular orbits. The modern model of the atom describes the positions of electrons in an atom in terms of probabilities. An electron can potentially be found at any distance from the nucleus, but, depending on its energy level, exists more frequently in certain regions around the nucleus than others; this pattern is referred to as its atomic orbital. The orbitals come in a variety of shapes-sphere, dumbbell, torus, etc.-with the nucleus in the middle.[29]

See also


  1. ^ Berryman, Sylvia, "Ancient Atomism", Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.) [1]
  2. ^ Pullman, Bernard (1998). The Atom in the History of Human Thought. Oxford, England: Oxford University Press. pp. 31–33. ISBN 978-0-19-515040-7.
  3. ^ a b Kenny, Anthony (2004). Ancient Philosophy. A New History of Western Philosophy. 1. Oxford, England: Oxford University Press. pp. 26–28. ISBN 0-19-875273-3.
  4. ^ a b c d e f g h i j Pyle, Andrew (2010). "Atoms and Atomism". In Grafton, Anthony; Most, Glenn W.; Settis, Salvatore. The Classical Tradition. Cambridge, Massachusetts and London, England: The Belknap Press of Harvard University Press. pp. 103–104. ISBN 978-0-674-03572-0.
  5. ^ a b c d Cohen, Henri; Lefebvre, Claire, eds. (2017). Handbook of Categorization in Cognitive Science (Second ed.). Amsterdam, The Netherlands: Elsevier. p. 427. ISBN 978-0-08-101107-2.
  6. ^ Weisstein, Eric W. "Lavoisier, Antoine (1743-1794)". Retrieved 2009-08-01.
  7. ^ Proust, Joseph Louis. "Researches on Copper", excerpted from Ann. chim. 32, 26-54 (1799) [as translated and reproduced in Henry M. Leicester and Herbert S. Klickstein, A Source Book in Chemistry, 1400–1900 (Cambridge, Massachusetts: Harvard, 1952)]. Retrieved on August 29, 2007.
  8. ^ Andrew G. van Melsen (1952). From Atomos to Atom. Dover Publications. p. 137. ISBN 978-0-486-49584-2.
  9. ^ a b Dalton, John. "On the Absorption of Gases by Water and Other Liquids", in Memoirs of the Literary and Philosophical Society of Manchester. 1803. Retrieved on August 29, 2007.
  10. ^ Johnson, Chris. "Avogadro - his contribution to chemistry". Archived from the original on 2002-07-10. Retrieved 2009-08-01.
  11. ^ Alan J. Rocke (1984). Chemical Atomism in the Nineteenth Century. Columbus: Ohio State University Press.
  12. ^ Avogadro, Amedeo (1811). "Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter into These Compounds". Journal de Physique. 73: 58–76.
  13. ^ Einstein, A. (1905). "Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen". Annalen der Physik. 322 (8): 549–560. Bibcode:1905AnP...322..549E. doi:10.1002/andp.19053220806. hdl:10915/2785.
  14. ^ Thomson, J.J. (1897). "Cathode rays" ([facsimile from Stephen Wright, Classical Scientific Papers, Physics (Mills and Boon, 1964)]). Philosophical Magazine. 44 (269): 293. doi:10.1080/14786449708621070.
  15. ^ Whittaker, E. T. (1951), A history of the theories of aether and electricity. Vol 1, Nelson, London
  16. ^ Thomson, J.J. (1904). "On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure". Philosophical Magazine. 7 (39): 237. doi:10.1080/14786440409463107.
  17. ^ Geiger, H (1910). "The Scattering of the α-Particles by Matter". Proceedings of the Royal Society. A 83: 492–504.
  18. ^ Rutherford, Ernest (1911). "The Scattering of α and β Particles by Matter and the Structure of the Atom" (PDF). Philosophical Magazine. 21 (4): 669. Bibcode:2012PMag...92..379R. doi:10.1080/14786435.2011.617037.
  19. ^ a b c Bohr, Niels (1913). "On the constitution of atoms and molecules" (PDF). Philosophical Magazine. 26 (153): 476–502. doi:10.1080/14786441308634993.
  20. ^ "Frederick Soddy, The Nobel Prize in Chemistry 1921". Nobel Foundation. Retrieved 2008-01-18.
  21. ^ Thomson, J.J. (1913). "Rays of positive electricity". Proceedings of the Royal Society. A 89 (607): 1–20. Bibcode:1913RSPSA..89....1T. doi:10.1098/rspa.1913.0057. [as excerpted in Henry A. Boorse & Lloyd Motz, The World of the Atom, Vol. 1 (New York: Basic Books, 1966)]. Retrieved on August 29, 2007.
  22. ^ Rutherford, Ernest (1919). "Collisions of alpha Particles with Light Atoms. IV. An Anomalous Effect in Nitrogen". Philosophical Magazine. 37 (222): 581. doi:10.1080/14786440608635919.
  23. ^ Chadwick, James (1932). "Possible Existence of a Neutron" (PDF). Nature. 129 (3252): 312. Bibcode:1932Natur.129Q.312C. doi:10.1038/129312a0.
  24. ^ Schrödinger, Erwin (1926). "Quantisation as an Eigenvalue Problem". Annalen der Physik. 81 (18): 109–139. Bibcode:1926AnP...386..109S. doi:10.1002/andp.19263861802.
  25. ^ Mahanti, Subodh. "Erwin Schrödinger: The Founder of Quantum Wave Mechanics". Archived from the original on 2009-04-17. Retrieved 2009-08-01.
  26. ^ Mahanti, Subodh. "Max Born: Founder of Lattice Dynamics". Archived from the original on 2009-01-22. Retrieved 2009-08-01.
  27. ^ Greiner, Walter. "Quantum Mechanics: An Introduction". Retrieved 2010-06-14.
  28. ^ Heisenberg, W. (1927). "Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik". Zeitschrift für Physik (in German). 43 (3–4): 172–198. Bibcode:1927ZPhy...43..172H. doi:10.1007/BF01397280.
  29. ^ Milton Orchin; Roger Macomber; Allan Pinhas; R. Wilson. "The Vocabulary and Concepts of Organic Chemistry, Second Edition," (PDF). Retrieved 2010-06-14.

Further reading

External links

  • Atomism by S. Mark Cohen.
  • Atomic Theory - detailed information on atomic theory with respect to electrons and electricity.
AP Chemistry

Advanced Placement Chemistry (AP Chemistry or AP Chem) is a course and examination offered by the College Board as a part of the Advanced Placement Program to give American and Canadian high school students the opportunity to demonstrate their abilities and earn college-level credit. AP Chemistry has the distinction of having the lowest known test participation rate, with 49.5% of AP Chemistry students taking the exam in one study. Another, smaller study found that 52.7% of students enrolled in AP Chemistry took their course's AP test.


An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).

Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects. Through the development of physics, atomic models have incorporated quantum principles to better explain and predict this behavior.

Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons. Protons and neutrons are called nucleons. More than 99.94% of an atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.

The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation.

The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element. The number of electrons influences the magnetic properties of an atom. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry.


Atomism (from Greek ἄτομον, atomon, i.e. "uncuttable, indivisible") is a natural philosophy that developed in several ancient traditions.

References to the concept of atomism and its atoms appeared in both ancient Greek and ancient Indian philosophical traditions. The ancient Greek atomists theorized that nature consists of two fundamental principles: atom and void. Unlike their modern scientific namesake in atomic theory, philosophical atoms come in an infinite variety of shapes and sizes, each indestructible, immutable and surrounded by a void where they collide with the others or hook together forming a cluster. Clusters of different shapes, arrangements, and positions give rise to the various macroscopic substances in the world.The particles of chemical matter for which chemists and other natural philosophers of the early 19th century found experimental evidence were thought to be indivisible, and therefore were given the name "atom", long used by the atomist philosophy. Although the connection to historical atomism is at best tenuous, elementary particles have become a modern analog of philosophical atoms.

Bohr model

In atomic physics, the Rutherford–Bohr model or Bohr model or Bohr diagram, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by revolving electrons —similar to the structure of the Solar System, but with attraction provided by electrostatic forces rather than gravity. After the cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911) came the Rutherford–Bohr model or just Bohr model for short (1913). The improvement to the Rutherford model is mostly a quantum physical interpretation of it.

The model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

The Bohr model is a relatively primitive model of the hydrogen atom, compared to the valence shell atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics or energy level diagrams before moving on to the more accurate, but more complex, valence shell atom. A related model was originally proposed by Arthur Erich Haas in 1910 but was rejected. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a full-blown quantum mechanics (1925) is often referred to as the old quantum theory.

Buddhist atomism

Buddhist atomism is a school of atomistic Buddhist philosophy that flourished on the Indian subcontinent during two major periods. During the first phase, which began to develop prior to the 6th century BCE, Buddhist atomism had a very qualitative, Aristotelian-style atomic theory. This form of atomism identifies four kinds of atoms, corresponding to the standard elements. Each of these elements has a specific property, such as solidity or motion, and performs a specific function in mixtures, such as providing support or causing growth. Like the Hindus and Jains, the Buddhists were able to integrate a theory of atomism with their logical presuppositions.

According to Noa Ronkin, this kind of atomism was developed in the Sarvastivada and Sautrantika schools for whom material reality can be: reduced to discrete momentary atoms, namely, the four primary elements. These momentary atoms, through their spatial arrangement and by their concatenation with prior and posterior atoms of the same type, create the illusion of persisting things as they appear in our everyday experience. Atomic reality is thus understood first and foremost as change, though not in the sense of a thing x transforming into y. That is, change itself is the very nature of atomic reality rather than its being made of enduring substances the qualities of which undergo change. Atoms that appear to endure are, in fact, a series of momentary events that ascend and fall in rapid succession and in accordance with causal relations. Unlike the atoms of the Vaifesika, the atoms of the Sarvastivada-Vaibhasika and the Sautrantika are not permanent: they come into being and cease from one moment to the next going through a process of birth, continuance, decay and destruction. Yet the material compounds that consist of these atoms are real, if only in the minimal, phenomenological sense.The second phase of Buddhist atomism, which flourished in the 7th century CE, was very different from the first. Indian Buddhist philosophers, including Dharmakirti and Dignāga, considered atoms to be point-sized, durationless, and made of energy. In discussing Buddhist atomism, Stcherbatsky writes:

... The Buddhists denied the existence of substantial matter altogether. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy... "Everything is evanescent," ... says the Buddhist, because there is no stuff ... Both systems [Sānkhya and later Indian Buddhism] share in common a tendency to push the analysis of Existence up to its minutest, last elements which are imagined as absolute qualities, or things possessing only one unique quality. They are called "qualities" (guna-dharma) in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed. Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality, ... and of the relation of Inference uniting them. There is in Sānkhya philosophy no separate existence of qualities. What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is called guna "quality", but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive ... or, more precisely, dynamic entities, although they are also called dharmas ("qualities").

Dalton Township, Ontario

The Township of Dalton was a municipality located in the northwest corner of the former Victoria County, now a geographic township in the city of Kawartha Lakes, in the Canadian province of Ontario. It was named after Dr. John Dalton (1766–1844), an English scientist who contributed to the foundations of atomic theory.

Dalton had an extensive history in logging and colonization along the Old Monck Road (Kawartha Lakes 45). Several Ghost villages dot the former township, many of them old logging/farming communities from the late 19th century. These include Ragged Rapids and Dartmoor. Some have survived since the logging days and remain inhabited, including Sadowa, Sebright, and Uphill. Back then, one of the most picturesque figures of the municipal history of the township was Joseph Thompson who was reeve for a quarter of a century. Thompson was a great hunter and many legends had been handed down concerning his prowess in the wilderness.

Dalton Transactions

Dalton Transactions is a peer-reviewed scientific journal publishing original (primary) research and review articles on all aspects of the chemistry of inorganic, bioinorganic, and organometallic compounds. It is published weekly by the Royal Society of Chemistry. The journal was named after the English chemist, John Dalton, best known for his work on modern atomic theory. Authors can elect to have accepted articles published as open access. The editor is Andrew Shore. Dalton Transactions was named a "rising star" by In-cites from Thomson Scientific in 2006.

History of thermodynamics

The history of thermodynamics is a fundamental strand in the history of physics, the history of chemistry, and the history of science in general. Owing to the relevance of thermodynamics in much of science and technology, its history is finely woven with the developments of classical mechanics, quantum mechanics, magnetism, and chemical kinetics, to more distant applied fields such as meteorology, information theory, and biology (physiology), and to technological developments such as the steam engine, internal combustion engine, cryogenics and electricity generation. The development of thermodynamics both drove and was driven by atomic theory. It also, albeit in a subtle manner, motivated new directions in probability and statistics; see, for example, the timeline of thermodynamics.

John Dalton

John Dalton FRS (; 6 September 1766 – 27 July 1844) was an English chemist, physicist, and meteorologist. He is best known for introducing the atomic theory into chemistry, and for his research into colour blindness, sometimes referred to as Daltonism in his honour.

Law of multiple proportions

In chemistry, the law of multiple proportions is one of the basic laws of stoichiometry used to establish the atomic theory, alongside the law of conservation of mass (matter) and the law of definite proportions. It is sometimes called Dalton's Law after its discoverer, the British chemist John Dalton, who published it in the first part of the first volume of his "New System of Chemical Philosophy" (1808). Here is the statement of the law:

If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.For example, Dalton knew that the element carbon forms two oxides by combining with oxygen in different proportions. A fixed mass of carbon, say 100 grams, may react with 133 grams of oxygen to produce one oxide, or with 266 grams of oxygen to produce the other. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 = 2:1, a ratio of small whole numbers. Dalton interpreted this result in his atomic theory by proposing (correctly in this case) that the two oxides have one and two oxygen atoms respectively for each carbon atom. In modern notation the first is CO (carbon monoxide) and the second is CO2 (carbon dioxide).

John Dalton first expressed this observation in 1804. A few years previously, the French chemist Joseph Proust had proposed the law of definite proportions, which expressed that the elements combined to form compounds in certain well-defined proportions, rather than mixing in just any proportion; and Antoine Lavoisier proved the law of conservation of mass, which helped out Dalton. Careful study of the actual numerical values of these proportions led Dalton to propose his law of multiple proportions. This was an important step toward the atomic theory that he would propose later that year, and it laid the basis for chemical formulas for compounds.

Another example of the law can be seen by comparing ethane (C2H6) with propane (C3H8). The weight of hydrogen which combines with 1 g carbon is 0.252 g in ethane and 0.224 g in propane. The ratio of those weights is 1.125, which can be expressed as the ratio of two small numbers 9:8.


Leucippus (; Greek: Λεύκιππος, Leúkippos; fl. 5th cent. BCE) is reported in some ancient sources to have been a philosopher who was the earliest Greek to develop the theory of atomism—the idea that everything is composed entirely of various imperishable, indivisible elements called atoms. Leucippus often appears as the master to his pupil Democritus, a philosopher also touted as the originator of the atomic theory. However, a brief notice in Diogenes Laërtius’s life of Epicurus says that on the testimony of Epicurus, Leucippus never existed. As the philosophical heir of Democritus, Epicurus's word has some weight, and indeed a controversy over this matter raged in German scholarship for many years at the close of the 19th century. Furthermore, in his Corpus Democriteum, Thrasyllus of Alexandria, an astrologer and writer living under the emperor Tiberius (14–37 CE), compiled a list of writings on atomism that he attributed to Democritus to the exclusion of Leucippus. The present consensus among the world's historians of philosophy is that this Leucippus is historical. The matter must remain moot unless more information is forthcoming from the record.

Leucippus was most likely born in Miletus, although Abdera and Elea are also mentioned as possible birthplaces.


Mochus (Greek: Μωχός), also known as Mochus of Sidon and Mochus the Phoenician, is listed by Diogenes Laërtius along with Zalmoxis the Thracian and Atlas of Mauretania, as a proto-philosopher. Athenaeus claimed that he authored a work on the history of Phoenicia. Strabo, on the authority of Posidonius, speaks of one Mochus or Moschus of Sidon as the author of the atomic theory and says that he was more ancient than the Trojan war. He is also referred to by Josephus, Tatian, and Eusebius.According to Robert Boyle, the father of modern chemistry, "‘Learned men attribute the devising of the atomical hypothesis to one Moschus a Phenician". Isaac Newton, Isaac Causabon, John Selden, Johannes Arcerius, Henry More, and Ralph Cudworth also credit Mochus of Sidon as the author of the atomic theory and some of them tried to identify Mochus with Moses the Israelite lawbringer.

Occult Chemistry

Occult Chemistry: Investigations by Clairvoyant Magnification into the Structure of the Atoms of the Periodic Table and Some Compounds (originally subtitled A Series of Clairvoyant Observations on the Chemical Elements) is a book written by Annie Besant and C.W. Leadbeater, who were all members of the Theosophical Society based in Adyar, India. Besant was at the time the President of the Society having succeeded Henry Olcott after his death in 1907.

Superseded theories in science

In science, a theory is superseded or becomes obsolete when a scientific consensus once widely accepted it, but current science considers it an inadequate, incomplete, or simply false description of reality. Such labels do not cover protoscientific or fringe science theories that have never had broad support within the scientific community. Furthermore, superseded or obsolete theories exclude theories that were never widely accepted by the scientific community. Some theories that were only supported under specific political authorities, such as Lysenkoism, may also be described as obsolete or superseded.

All of Newtonian physics is so satisfactory for most purposes that it is more widely used except at velocities that are a significant fraction of the speed of light, and simpler Newtonian but not relativistic mechanics is usually taught in schools. Another case is the belief that the Earth is approximately flat. For centuries, people have known that a flat Earth model produces errors in long-distance calculations, but considering local-scale areas as flat for the purposes of mapping and surveying does not introduce significant errors.

In some cases, a theory or idea is found baseless and is simply discarded. For example, the phlogiston theory was entirely replaced by the quite different concept of energy and related laws. In other cases an existing theory is replaced by a new theory that retains significant elements of the earlier theory; in these cases, the older theory is often still useful for many purposes, and may be more easily understood than the complete theory and lead to simpler calculations. An example of this is the use of Newtonian physics, which differs from the currently accepted relativistic physics by a factor that is negligibly small at velocities much lower than that of light.


Swerve may refer to:

Turning an automobile sharply to avoid a road hazard

Clinamen, a concept in early atomic theory

The curved flight of a spinning object due to the Magnus effect

Twin/Tone Records

Twin/Tone Records was a independent record label based in Minneapolis, Minnesota, which operated from 1977 until 1994. It was the original home of influential Minnesota bands the Replacements and Soul Asylum and was instrumental in helping the Twin Cities music scene achieve national attention in the 1980s. Along with other independent American labels such as SST Records, Touch and Go Records, and Dischord, Twin/Tone helped to spearhead the nationwide network of underground bands that formed the pre-Nirvana indie-rock scene. These labels presided over the shift from the hardcore punk that then dominated the American underground scene to the more diverse styles of alternative rock that were emerging.Twin/Tone originated in the Minneapolis punk rock scene, which included venues like Jay's Longhorn Bar. The label was begun by Peter Jesperson, local music and sports writer Charley Hallman, and Paul Stark. Releases by the pop/rock group The Suburbs were both Twin/Tone's first release (The Suburbs EP in 1978) and its last (Viva! Suburbs! in 1994). Jesperson signed the Replacements to the label immediately after the band's debut at the Longhorn Bar in Minneapolis. By 1984, the label had released 41 records and grown large enough to support three paid staff members, with its biggest-selling records including the Suburbs' debut and the first two discs by the Replacements. Other groups that signed with Twin/Tone include the Magnolias, Babes in Toyland, Information Society, Agitpop, the Jayhawks, Poster Children, Soul Asylum, the Wallets, Curtiss A, and Pennsylvania-based Ween. British alternative-rock musician Robyn Hitchcock also released his 1990 solo album Eye through the label.

By 1994, Twin/Tone had released more than 300 records by 100 bands and had begun to develop an umbrella relationship with several smaller, mostly Minnesota-based indie labels, including:

In 1995, Twin/Tone was recognized as a "significant regional label" by Billboard magazine. In 1998, Stark decided to stop releasing physical product in favor of digital media. The company is currently described as being "in mothballs", releasing only limited amounts of out-of-print material on custom-burned CDs, though some of the more significant material was licensed to Restless Records, part of Rykodisc. Hallman died in 2015. In 2017, the label was revived by Stark and Jesperson to release the Suicide Commandos comeback album Time Bomb.

Uncleftish Beholding

"Uncleftish Beholding" (1989) is a short text by Poul Anderson designed to illustrate what English might look like without its large number of loanwords from languages such as French, Greek, and Latin. Written in a form of "Anglish," the work explains atomic theory using Germanic words almost exclusively and coining new words when necessary; many of these new words have cognates in modern German, an important scientific language in its own right. The title phrase uncleftish beholding calques "atomic theory."To illustrate, the text begins:

For most of its being, mankind did not know what things are made of, but could only guess. With the growth of worldken, we began to learn, and today we have a beholding of stuff and work that watching bears out, both in the workstead and in daily life.

It goes on to define firststuffs (chemical elements), such as waterstuff (hydrogen), sourstuff (oxygen), and ymirstuff (uranium), as well as bulkbits (molecules), bindings (compounds), and several other terms important to uncleftish worldken (atomic physics). Wasserstoff and Sauerstoff are the modern German words for hydrogen and oxygen, and in Dutch the modern equivalents are waterstof and zuurstof. Sunstuff refers to helium, which derives from ἥλιος, the Ancient Greek word for "sun." Ymirstuff references Ymir, a giant in Norse mythology similar to Uranus in Greek mythology.

The vocabulary used in Uncleftish Beholding does not completely derive from Anglo-Saxon. Around, from Old French reond (Modern French rond), completely displaced Old English ymbe (cognate to German um) and left no "native" English word for this concept. The text also contains the French-derived words rest, ordinary and sort.

The text gained increased exposure and popularity after being circulated around the Internet, and has served as inspiration for some inventors of Germanic English conlangs. Douglas Hofstadter, in discussing the piece in his book Le Ton beau de Marot, jocularly refers to the use of only Germanic roots for scientific pieces as "Ander-Saxon."

Interestingly, the title of the work points out a flaw in the naming of the atom, as by 6 August 1945, atoms had already been cleft, proving them "cleftish."


Vaisheshika or Vaiśeṣika (Sanskrit: वैशेषिक) is one of the six orthodox schools of Hindu philosophy (Vedic systems) from ancient India. In its early stages, the Vaiśeṣika was an independent philosophy with its own metaphysics, epistemology, logic, ethics, and soteriology. Over time, the Vaiśeṣika system became similar in its philosophical procedures, ethical conclusions and soteriology to the Nyāya school of Hinduism, but retained its difference in epistemology and metaphysics.

The epistemology of Vaiśeṣika school of Hinduism, like Buddhism, accepted only two reliable means to knowledge: perception and inference. Vaiśeṣika school and Buddhism both consider their respective scriptures as indisputable and valid means to knowledge, the difference being that the scriptures held to be a valid and reliable source by Vaiśeṣikas were the Vedas.

Vaisheshika school is known for its insights in naturalism. It is a form of atomism in natural philosophy. It postulated that all objects in the physical universe are reducible to paramāṇu (atoms), and one's experiences are derived from the interplay of substance (a function of atoms, their number and their spatial arrangements), quality, activity, commonness, particularity and inherence. According to Vaiśeṣika school, knowledge and liberation were achievable by a complete understanding of the world of experience.Vaiśeṣika darshana was founded by Kaṇāda Kashyapa around the 6th to 2nd century BC.

William Higgins (chemist)

William Higgins (1763 – June 1825), an Irish chemist, was one of the early proponents of atomic theory. Known mainly for his speculative ideas on chemical combination, William Higgins is popular for the insights his life offers into the emergence of chemistry as a career during the British Industrial Revolution. Despite an evident charm, his erratic behavior and tendency to indulge personal animosities prevented him from engaging the affections of London society. Instead he found refuge in a succession of government-supported chemical positions in Dublin. Thanks to the combination of such scientific opportunities with family resources, he became a very wealthy man.

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