Standard atomic weight

The standard atomic weight (Ar, standard, a relative atomic mass) is the atomic weight (Ar) of a chemical element, as appearing and met in the earthly environment. It reflects the variance of natural isotopes (and so weight differences) of an element. Values are defined by (restricted to) the IUPAC (CIAAW) definition of natural, stable, terrestrial sources. It is the most common and practical atomic weight used, for example to determine molar mass.

The specified definition is to use many representative sources (samples) from the Earth, so that the value can widely be used as 'the' atomic weight for real life substances—for example, in pharmaceuticals and scientific research. Atomic weights are specific to single sources and samples of an element, such as the atomic weight of carbon in a particular bone from a particular archeological site. Standard atomic weight generalizes such values to the range of atomic weights which a chemist might expect to derive from many random samples from Earth. This range is the cause of the interval notation in some standard atomic weight values.

Out of the 118 known chemical elements, 84 are stable and have this Earth-environment based value. Typically, such a value is, for example helium: Ar, standard(He) = 4.002602(2). The "(2)" indicates the uncertainty in the last digit shown, to read 4.002602 ±0.000002. IUPAC also publishes abridged values, rounded to five significant figures. For helium, Ar, abridged(He) = 4.0026.

For twelve elements the samples diverge on this value, because their sample sources have had a different decay history. For example, thallium (Tl) in sedimentary rocks has a different isotopic composition than in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: Ar, standard(Tl) = [204.38, 204.39]. With such an interval, for less demanding situations, IUPAC also publishes an conventional value. For thallium, Ar, conventional(Tl) = 204.38.

CIAAW 2013 - Standard atomic weight for cupper (29, Cu)
Example: copper in terrestrial sources. Two isotopes are present: copper-63 (62.9 u) and copper-65 (64.9 u), in abundances 69% + 31%. The standard atomic weight (Ar, standard) for copper is the average, taken their abundance into account, and then divided by the standardised ​112 12C unit.[1]


IUPAC Periodic Table of the Elements 2011
Excerpt of an IUPAC Periodic Table showing the interval notation of the standard atomic weights of boron, carbon, and nitrogen (Chemistry International, IUPAC). Example: the pie chart for boron shows it to be composed of about 20% 10B and 80% 11B. This isotope mix causes the atomic weight of ordinary Earthly boron samples to be expected to fall within the interval 10.806 to 10.821. and this interval is the standard atomic weight. Boron samples from unusual sources, particularly non-terrestrial sources, might have measured atomic weights that fall outside this range. Atomic weight and relative atomic mass are synonyms.

The standard atomic weight is thus a more special value of the relative atomic mass. It is defined as the "recommended values" of relative atomic masses of sources in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances. (CIAAW)[2] In general, values from different sources are subject to natural variation due to a different radioactive history of sources. Thus, standard atomic weights are an expectation range of atomic weights from a range of samples or sources. By limiting the sources to terrestrial origin only, the CIAAW-determined values have less variance, and are a more precise value for relative atomic masses (atomic weights) actually found and used in worldly materials.

The CIAAW-published values are used and sometimes lawfully required in mass calculations. The values have an uncertainty (noted in brackets), or are an expectation interval (see example in illustration immediately above). This uncertainty reflects natural variability in isotopic distribution for an element, rather than uncertainty in measurement (which is much smaller with quality instruments).[3]

Although there is an attempt to cover the range of variability on Earth with standard atomic weight figures, there are known cases of mineral samples which contain elements with atomic weights that are outliers from the standard atomic weight range.[4]

For synthetic elements the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleon count of the most stable isotope (i.e., the isotope with the longest half-life) is listed in brackets, in place of the standard atomic weight.

When the term "atomic weight" is used in chemistry, usually it is the more specific standard atomic weight that is implied. It is standard atomic weights that are used in periodic tables and many standard references in ordinary terrestrial chemistry.

Lithium represents a unique case where the natural abundances of the isotopes have in some cases been found to have been perturbed by human isotopic separation activities to the point of affecting the uncertainty in its standard atomic weight, even in samples obtained from natural sources, such as rivers.[citation needed, dubious]

Terrestrial definition

An example of why “conventional terrestrial sources" must be specified in giving standard atomic weight values is the element argon. Between locations in the Solar System, the atomic weight of argon varies as much as 10%, due to extreme variance in isotopic composition. Where the major source of argon is the decay of 40
in rocks, 40
will be the dominant isotope. Such locations include the planets Mercury and Mars, and the moon Titan. On Earth the ratios of the three isotopes 36Ar : 38Ar : 40Ar are approximately 5 : 1 : 1600, giving terrestrial argon a standard atomic weight of 39.948(1). This atomic weight is larger than that of the next element potassium, causing confusion in the days when the places of elements in the periodic table was largely determined according to atomic weight.

However, such is not the case in the rest of the universe. Argon produced directly by stellar nucleosynthesis, is dominated by the alpha-process nuclide 36
. Correspondingly, solar argon contains 84.6% 36
(according to solar wind measurements),[5] and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is 8400 : 1600 : 1.[6] The atomic weight of argon in the Sun and most of the universe, therefore, would be only approximately 36.3.[7]

Causes of uncertainty on Earth

Famously, the published atomic weight value comes with an uncertainty. This uncertainty (and related: precision) follows from its definition, the source being "terrestrial and stable". Systematic causes for uncertainty are:

  1. Measurement limits. As always, the physical measurement is never finite. There is always more detail to be found and read. This applies to every single, pure isotope found. For example, today the mass of the main natural fluorine isotope can be measured to the accuracy of eleven decimal places: 18.998403163(6). But a still more precise measurement system could become available, producing more decimals.
  2. Imperfect mixtures of isotopes. In the samples taken and measured the mix (relative abundance) of those isotopes may vary. For example copper. While in general its two isotopes make out 69.15% and 30.85% each of all copper found, the natural sample being measured can have had an incomplete 'stirring' and so the percentages are different. The precision is improved by measuring more samples of course, but there remains this cause of uncertainty. (Example: lead samples vary so much, it can not be noted more precise than four figures: 207.2)
  3. Earthly sources with a different history. A source is the greater area being researched, for example 'ocean water' or 'volcanic rock' (as opposed to a 'sample': the single heap of material being investigated). It appears that some elements have a different isotopic mix per source. For example, thallium in igneous rock has more lighter isotopes, while in sedimentary rock it has more heavy isotopes. There is no Earthly mean number. These elements show the interval notation: Ar, standard(Tl) = [204.38204.39]. For practical reasons, a simplified 'conventional' number is published too (for Tl: 204.38).

These three uncertainties are accumulative. The published value is a result of all these.

Determination of relative atomic mass

Modern relative atomic masses (a term specific to a given element sample) are calculated from measured values of atomic mass (for each nuclide) and isotopic composition of a sample. Highly accurate atomic masses are available[8][9] for virtually all non-radioactive nuclides, but isotopic compositions are both harder to measure to high precision and more subject to variation between samples.[10][11] For this reason, the relative atomic masses of the 22 mononuclidic elements (which are the same as the isotopic masses for each of the single naturally occurring nuclides of these elements) are known to especially high accuracy. For example, there is an uncertainty of only one part in 38 million for the relative atomic mass of fluorine, a precision which is greater than the current best value for the Avogadro constant (one part in 20 million).

Isotope Atomic mass[9] Abundance[10]
Standard Range
28Si 27.976 926 532 46(194) 92.2297(7)% 92.21–92.25%
29Si 28.976 494 700(22) 4.6832(5)% 4.67–4.69%
30Si 29.973 770 171(32) 3.0872(5)% 3.08–3.10%

The calculation is exemplified for silicon, whose relative atomic mass is especially important in metrology. Silicon exists in nature as a mixture of three isotopes: 28Si, 29Si and 30Si. The atomic masses of these nuclides are known to a precision of one part in 14 billion for 28Si and about one part in one billion for the others. However the range of natural abundance for the isotopes is such that the standard abundance can only be given to about ±0.001% (see table). The calculation is

Ar(Si) = (27.97693 × 0.922297) + (28.97649 × 0.046832) + (29.97377 × 0.030872) = 28.0854

The estimation of the uncertainty is complicated,[12] especially as the sample distribution is not necessarily symmetrical: the IUPAC standard relative atomic masses are quoted with estimated symmetrical uncertainties,[13] and the value for silicon is 28.0855(3). The relative standard uncertainty in this value is 1×10–5 or 10 ppm. To further reflect this natural variability, in 2010, IUPAC made the decision to list the relative atomic masses of 10 elements as an interval rather than a fixed number.[14]

Naming controversy

The use of the name "atomic weight" has attracted a great deal of controversy among scientists.[15] Objectors to the name usually prefer the term "relative atomic mass" (not to be confused with atomic mass). The basic objection is that atomic weight is not a weight, that is the force exerted on an object in a gravitational field, measured in units of force such as the newton or poundal.

In reply, supporters of the term "atomic weight" point out (among other arguments)[15] that

  • the name has been in continuous use for the same quantity since it was first conceptualized in 1808;[16]
  • for most of that time, atomic weights really were measured by weighing (that is by gravimetric analysis) and the name of a physical quantity should not change simply because the method of its determination has changed;
  • the term "relative atomic mass" should be reserved for the mass of a specific nuclide (or isotope), while "atomic weight" be used for the weighted mean of the atomic masses over all the atoms in the sample;
  • it is not uncommon to have misleading names of physical quantities which are retained for historical reasons, such as

It could be added that atomic weight is often not truly "atomic" either, as it does not correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense.

Published values

IUPAC publishes one formal value for each stable element, called the standard atomic weight.[17][18] Any updates are published biannually (in uneven years). The last change was published in 2015, setting a new value for ytterbium[19] Per 2017, 14 atomic weights were changed, including argon changing from single number to interval value.[20][21]

The value published can have and uncertainty be an interval like for neon: 20.1797(6), or can be an interval, like for boron: [10.806, 10.821].

Next to these 84 values, IUPAC also publishes abridged values (up to five digits per number only), and for the twelve interval values, conventional values (single number values).

Symbol Ar is a relative atomic mass, for example from a specific sample. To be specific, the standard atomic weight can be noted as Ar, standard(E), where (E) is the element symbol.

Abridged atomic weight

The abridged atomic weight, also published by CIAAW, is derived from the standard atomic weight reducing the numbers to five digits (five significant figures). The name does not say 'rounded'.

Interval borders are rounded downwards for the first (lowmost) border, and upwards for the upward (upmost) border. This way, the more precise original interval is fully covered.[22]


  • Calcium: Ar, standard(Ca) = 40.078(4)Ar, abridged(Ca) = 40.078
  • Helium: Ar, standard(He) = 4.002602(2)Ar, abridged(He) = 4.0026
  • Hydrogen: Ar, standard(H) = [1.00784, 1.00811]Ar, abridged(H) = [1.0078, 1.0082]

Conventional atomic weight

Twelve chemical elements have a standard atomic weight that is defined not as a single number, but as an interval. For example, hydrogen has Ar, standard(H) = [1.00 784, 1.00811]. This notation states that the various sources on Earth have substantially different isotopic constitutions, and uncertainties are incorporated in the two numbers. For these elements, there is not an 'Earth average' constitution, and the 'right' value is not its middle (that would be 1.007975 for hydrogen, with an uncertainty of (±0.000135) that would make it just cover the interval). However, for situations where a less precise value is acceptable, CIAAW has published a single-number conventional atomic weight that can be used for example in trade. For hydrogen, Ar, conventional(H) = 1.008. The twelve elements are: hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, bromine and thallium.[23]

A formal short atomic weight

By using the abridged value, and the conventional value for the twelve interval values, a short IUPAC-defined value (5 digits plus uncertainty) can be given for all stable elements. In many situations, and in periodic tables, this may be sufficiently detailed.[24]

Overview: formal values of the standard atomic weight[1]
Element Ar, standard Ar, std abridged[18] Ar, std conventional[22] Ar, std formal short[23] Mass number
(most stable isotope)
hydrogen 1H [1.007841.00811] [1.00781.0082] 1.008 1.008
nitrogen 7N [14.0064314.00728] [14.00614.008] 14.007 14.007
fluorine 9F 18.998403163(6) 18.998 18.998
calcium 20Ca 40.078(4) 40.078(4) 40.078(4)
technetium 43Tc (none) [98]

List of atomic weights

Standard atomic weight of the elements (IUPAC 2013,[1] 2015[25], 2017)
Z Symbol Name Ar, standard abridged conventional → formal, short note
1 H hydrogen [1.007841.00811] [1.00781.0082] 1.008 1.008
2 He helium 4.002602(2) 4.0026 4.0026
3 Li lithium [6.9386.997] [6.9386.997] 6.94 6.94
4 Be beryllium 9.0121831(5) 9.0122 9.0122
5 B boron [10.80610.821] [10.80610.821] 10.81 10.81
6 C carbon [12.009612.0116] [12.00912.012] 12.011 12.011
7 N nitrogen [14.0064314.00728] [14.00614.008] 14.007 14.007
8 O oxygen [15.9990315.99977] [15.99916.000] 15.999 15.999
9 F fluorine 18.998403163(6) 18.998 18.998
10 Ne neon 20.1797(6) 20.180 20.180
11 Na sodium 22.98976928(2) 22.990 22.990
12 Mg magnesium [24.30424.307] [24.30424.307] 24.305 24.305
13 Al aluminium 26.9815384(3) 26.982 26.982
14 Si silicon [28.08428.086] [28.08428.086] 28.085 28.085
15 P phosphorus 30.973761998(5) 30.974 30.974
16 S sulfur [32.05932.076] [32.05932.076] 32.06 32.06
17 Cl chlorine [35.44635.457] [35.44635.457] 35.45 35.45
18 Ar argon [39.79239.963] [39.79239.963] 39.948 39.948
19 K potassium 39.0983(1) 39.098 39.098
20 Ca calcium 40.078(4) 40.078(4) 40.078(4)
21 Sc scandium 44.955908(5) 44.956 44.956
22 Ti titanium 47.867(1) 47.867 47.867
23 V vanadium 50.9415(1) 50.942 50.942
24 Cr chromium 51.9961(6) 51.996 51.996
25 Mn manganese 54.938043(2) 54.938 54.938
26 Fe iron 55.845(2) 55.845(2) 55.845(2)
27 Co cobalt 58.933194(3) 58.933 58.933
28 Ni nickel 58.6934(4) 58.693 58.693
29 Cu copper 63.546(3) 63.546(3) 63.546(3)
30 Zn zinc 65.38(2) 65.38(2) 65.38(2)
31 Ga gallium 69.723(1) 69.723 69.723
32 Ge germanium 72.630(8) 72.630(8) 72.630(8)
33 As arsenic 74.921595(6) 74.922 74.922
34 Se selenium 78.971(8) 78.971(8) 78.971(8)
35 Br bromine [79.90179.907] [79.90179.907] 79.904 79.904
36 Kr krypton 83.798(2) 83.798(2) 83.798(2)
37 Rb rubidium 85.4678(3) 85.468 85.468
38 Sr strontium 87.62(1) 87.62 87.62
39 Y yttrium 88.90584(1) 88.906 88.906
40 Zr zirconium 91.224(2) 91.224(2) 91.224(2)
41 Nb niobium 92.90637(1) 92.906 92.906
42 Mo molybdenum 95.95(1) 95.95 95.95
43 Tc technetium - -
44 Ru ruthenium 101.07(2) 101.07(2) 101.07(2)
45 Rh rhodium 102.90549(2) 102.91 102.91
46 Pd palladium 106.42(1) 106.42 106.42
47 Ag silver 107.8682(2) 107.87 107.87
48 Cd cadmium 112.414(4) 112.41 112.41
49 In indium 114.818(1) 114.82 114.82
50 Sn tin 118.710(7) 118.71 118.71
51 Sb antimony 121.760(1) 121.76 121.76
52 Te tellurium 127.60(3) 127.60(3) 127.60(3)
53 I iodine 126.90447(3) 126.90 126.90
54 Xe xenon 131.293(6) 131.29 131.29
55 Cs caesium 132.90545196(6) 132.91 132.91
56 Ba barium 137.327(7) 137.33 137.33
57 La lanthanum 138.90547(7) 138.91 138.91
58 Ce cerium 140.116(1) 140.12 140.12
59 Pr praseodymium 140.90766(1) 140.91 140.91
60 Nd neodymium 144.242(3) 144.24 144.24
61 Pm promethium - -
62 Sm samarium 150.36(2) 150.36(2) 150.36(2)
63 Eu europium 151.964(1) 151.96 151.96
64 Gd gadolinium 157.25(3) 157.25(3) 157.25(3)
65 Tb terbium 158.925354(8) 158.93 158.93
66 Dy dysprosium 162.500(1) 162.50 162.50
67 Ho holmium 164.930328(7) 164.93 164.93
68 Er erbium 167.259(3) 167.26 167.26
69 Tm thulium 168.934218(6) 168.93 168.93
70 Yb ytterbium 173.045(10) 173.05 173.05 2015[25]
71 Lu lutetium 174.9668(1) 174.97 174.97
72 Hf hafnium 178.49(2) 178.49(2) 178.49(2)
73 Ta tantalum 180.94788(2) 180.95 180.95
74 W tungsten 183.84(1) 183.84 183.84
75 Re rhenium 186.207(1) 186.21 186.21
76 Os osmium 190.23(3) 190.23(3) 190.23(3)
77 Ir iridium 192.217(2) 192.22 192.22
78 Pt platinum 195.084(9) 195.08 195.08
79 Au gold 196.966570(4) 196.97 196.97
80 Hg mercury 200.592(3) 200.59 200.59
81 Tl thallium [204.382204.385] [204.38204.39] 204.38 204.38
82 Pb lead 207.2(1) 207.2 207.2
83 Bi bismuth 208.98040(1) 208.98 208.98
84 Po polonium - -
85 At astatine - -
86 Rn radon - -
87 Fr francium - -
88 Ra radium - -
89 Ac actinium - -
90 Th thorium 232.0377(4) 232.04 232.04
91 Pa protactinium 231.03588(1) 231.04 231.04
92 U uranium 238.02891(3) 238.03 238.03
93 Np neptunium - -
94 Pu plutonium - -
95 Am americium - -
96 Cm curium - -
97 Bk berkelium - -
98 Cf californium - -
99 Es einsteinium - -
100 Fm fermium - -
101 Md mendelevium - -
102 No nobelium - -
103 Lr lawrencium - -
104 Rf rutherfordium - -
105 Db dubnium - -
106 Sg seaborgium - -
107 Bh bohrium - -
108 Hs hassium - -
109 Mt meitnerium - -
110 Ds darmstadtium - -
111 Rg roentgenium - -
112 Cn copernicium - -
113 Nh nihonium - -
114 Fl flerovium - -
115 Mc moscovium - -
116 Lv livermorium - -
117 Ts tennessine - -
118 Og oganesson - -

In the periodic table

See also


  1. ^ a b c d Meija, J.; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. ^ IUPAC Definition of Standard Atomic Weight
  3. ^ Wieser, M. E (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)" (PDF). Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.
  4. ^ IUPAC Goldbook says Definition of standard atomic weights: "Recommended values of relative atomic masses of the elements revised biennially by the IUPAC Commission on Atomic Weights and Isotopic Abundances and applicable to elements in any normal sample with a high level of confidence. A normal sample is any reasonably possible source of the element or its compounds in commerce for industry and science and has not been subject to significant modification of isotopic composition within a geologically brief period."
  5. ^ Lodders, K. (2008). "The solar argon abundance". Astrophysical Journal. 674 (1): 607–611. arXiv:0710.4523. Bibcode:2008ApJ...674..607L. doi:10.1086/524725.
  6. ^ Cameron, A. G. W. (1973). "Elemental and isotopic abundances of the volatile elements in the outer planets". Space Science Reviews. 14 (3–4): 392–400. Bibcode:1973SSRv...14..392C. doi:10.1007/BF00214750.
  7. ^ This can be determined from the preceding figures per the definition of atomic weight and WP:CALC
  8. ^ National Institute of Standards and Technology. Atomic Weights and Isotopic Compositions for All Elements.
  9. ^ a b Wapstra, A.H.; Audi, G.; Thibault, C. (2003), The AME2003 Atomic Mass Evaluation (Online ed.), National Nuclear Data Center. Based on:
  10. ^ a b Rosman, K. J. R.; Taylor, P. D. P. (1998), "Isotopic Compositions of the Elements 1997" (PDF), Pure and Applied Chemistry, 70 (1): 217–35, doi:10.1351/pac199870010217
  11. ^ Coplen, T. B.; et al. (2002), "Isotopic Abundance Variations of Selected Elements" (PDF), Pure and Applied Chemistry, 74 (10): 1987–2017, doi:10.1351/pac200274101987
  12. ^ Meija, Juris; Mester, Zoltán (2008). "Uncertainty propagation of atomic weight measurement results". Metrologia. 45 (1): 53–62. Bibcode:2008Metro..45...53M. doi:10.1088/0026-1394/45/1/008.
  13. ^ Holden, Norman E. (2004). "Atomic Weights and the International Committee—A Historical Review". Chemistry International. 26 (1): 4–7.
  14. ^ IUPAC – International Union of Pure and Applied Chemistry: Atomic Weights of Ten Chemical Elements About to Change
  15. ^ a b de Bièvre, P.; Peiser, H. S. (1992). "'Atomic Weight'—The Name, Its History, Definition, and Units". Pure Appl. Chem. 64 (10): 1535–43. doi:10.1351/pac199264101535.
  16. ^ Dalton, John (1808). A New System of Chemical Philosophy. Manchester.
  17. ^ "Atomic Weights". Retrieved 2018-03-13.
  18. ^ a b IUPAC 2016, Table 1.
  19. ^ "Standard Atomic Weights 2015". Commission on Isotopic Abundances and Atomic Weights. 12 October 2015. Retrieved 18 February 2017.
  20. ^ "Standard atomic weights of 14 chemical elements revised". CIAAW. 2018-06-05. Retrieved 2019-02-02.
  21. ^ "Standard Atomic Weights of 14 Chemical Elements Revised". Chemistry International. 40 (4): 23–24. 2018. doi:10.1515/ci-2018-0409. ISSN 0193-6484.
  22. ^ a b IUPAC 2016, Table 2.
  23. ^ a b IUPAC 2016, Table 3.
  24. ^ IUPAC 2016, Table 2 and Table 3 combined.
  25. ^ a b "Standard Atomic Weight of Ytterbium Revised". Chemistry International. October 2015. p. 26. doi:10.1515/ci-2015-0512. eISSN 0193-6484. ISSN 0193-6484.
  26. ^ IUPAC 2016, Table 2, 3 combined; uncertainty removed.

External links

Atomic mass

The atomic mass (ma) is the mass of an atom. Its unit is the unified atomic mass units (abbr. u) where 1 unified atomic mass unit is defined as ​1⁄12 of the mass of a single carbon-12 atom, at rest. For atoms, the protons and neutrons of the nucleus account for nearly all of the total mass, and the atomic mass measured in u has nearly the same value as the mass number.

When divided by unified atomic mass units, or daltons (abbr. Da), to form a pure numeric ratio, the atomic mass of an atom becomes a dimensionless value called the relative isotopic mass (see section below). Thus, the atomic mass of a carbon-12 atom is 12 u (or 12 Da), but the relative isotopic mass of a carbon-12 atom is simply 12.

The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The dimensionless standard atomic weight instead refers to the average (mathematical mean) of atomic mass values of a typical naturally-occurring mixture of isotopes for a sample of an element. Atomic mass values are thus commonly reported to many more significant figures than atomic weights. Standard atomic weight is related to atomic mass by the abundance ranking of isotopes for each element. It is usually about the same value as the atomic mass of the most abundant isotope, other than what looks like (but is not actually) a rounding difference.

The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to binding energy mass loss (as per E = mc2).

Atomic number

The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.

The sum of the atomic number Z and the number of neutrons, N, gives the mass number A of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.

Atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.

The conventional symbol Z comes from the German word Zahl meaning number, which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.

Extended periodic table (detailed cells)

This is a large version of the extended periodic table of the chemical elements. For the natural isotopic composition of each element it shows name, atomic number, symbol, standard atomic weight (or atomic weight) and a link to the element's isotopes. It also has, by keyed markings, a notification of occurrence, state of matter and metallic character. It is named extended because it shows elements that are theoretical only (in period 8), and large means that the table might exceed page width. The presentation is wide which means that there are no table parts separated (as is more common in print).

The current standard table contains 7 periods, culminating in oganesson, which has atomic number 118. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior. If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing 18 elements with partially filled g-orbitals in each period. An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969.It is not known how many elements are physically possible, if period 8 is complete, or if there is a period 9.

Isotopes of aluminium

Aluminium or aluminum (13Al) has 25 known isotopes from 19Al to 43Al and 4 known isomers. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2 × 105 y) occur naturally, however 27Al has a natural abundance of >99.9%. Other than 26Al, all radioisotopes have half-lives under 7 minutes, most under a second. The standard atomic weight is 26.9815385(7). 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study the role of sediment transport, deposition, and storage, as well as burial times, and erosion, on 105 to 106 year time scales.Cosmogenic aluminium-26 was first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.

Isotopes of beryllium

Beryllium (4Be) has 12 known isotopes, but only one of these isotopes (9Be) is stable and a primordial nuclide. As such, beryllium is considered a monoisotopic element. It is also a mononuclidic element, because its other isotopes have such short half-lives that none are primordial and their abundance is very low (standard atomic weight is 9.0122). Beryllium is unique as being the only monoisotopic element with both an even number of protons and an odd number of neutrons. There are 25 other monoisotopic elements but all have odd atomic numbers, and even numbers of neutrons.

Of the 11 radioisotopes of beryllium, the most stable are 10Be with a half-life of 1.39 million years and 7Be with a half-life of 53.22 days. All other radioisotopes have half-lives under 13.85 seconds, most under 0.03 seconds. The least stable isotope is 6Be, with a half-life measured as 5.03 × 10−21 seconds.

The natural light-element ratio of equal proton and neutron numbers is prevented in beryllium by the extreme instability of 8Be toward alpha decay, which is favored due to the extremely tight binding of 4He nuclei. The half-life for the decay of 8Be is only 6.7(17)×10−17 seconds.

Beryllium is prevented from having a stable isotope with 4 protons and 6 neutrons by the very large mismatch in proton/neutron ratio for such a light element. Nevertheless, this isotope, 10Be, has a half-life of 1.39 million years, which indicates unusual stability for a light isotope with such a large neutron/proton imbalance. Still other possible beryllium isotopes have even more severe mismatches in neutron and proton number, and thus are even less stable.

Most 9Be in the universe is thought to be formed by cosmic ray nucleosynthesis from cosmic ray spallation in the period between the Big Bang and the formation of the solar system. The isotopes 7Be, with a half-life of 53.22 days, and 10Be are both cosmogenic nuclides because they are made on a recent timescale in the solar system by spallation, like 14C. These two radioisotopes of beryllium in the atmosphere track the sun spot cycle and solar activity, since this affects the magnetic field that shields the Earth from cosmic rays. The rate at which the short-lived 7Be is transferred from the air to the ground is controlled in part by the weather. 7Be decay in the sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment. Presence of 7Be in sediments is often used to establish that they are fresh, i.e. less than about 3–4 months in age, or about two half-lives of 7Be.

Isotopes of chlorine

Chlorine (17Cl) has 25 isotopes with mass numbers ranging from 28Cl to 52Cl and 2 isomers (34mCl and 38mCl). There are two stable isotopes, 35Cl (75.78%) and 37Cl (24.22%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is 36Cl, which has a half-life of 301,000 years. All other isotopes have half-lives under 1 hour, many less than one second. The shortest-lived are 29Cl and 30Cl, with half-lives less than 20 and 30 nanoseconds, respectively—the half-life of 28Cl is unknown.

Isotopes of curium

Curium (96Cm) is an artificial element with an atomic number of 96. Because it is an artificial element, a standard atomic weight cannot be given, and it has no stable isotopes. The first isotope synthesized was 242Cm in 1944, which has 146 neutrons.

There are 19 known radioisotopes with atomic masses ranging from 233Cm to 251Cm. There are also ten known nuclear isomers. The longest-lived isotope is 247Cm, with a half-life of 15.6 million years – several orders of magnitude longer than the half-life of all known nuclei of elements beyond curium in the periodic table. The longest-lived isomer is 246mCm with a half-life of 1.12 seconds.

Isotopes of einsteinium

Einsteinium (99Es) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in nuclear fallout from an H-bomb test) was 253Es in 1952. There are 19 known radioisotopes from 240Es to 258Es, and 3 nuclear isomers (250mEs, 254mEs, and 256mEs). The longest-lived isotope is 252Es with a half-life of 471.7 days, or around 1.293 years.

Isotopes of fermium

Fermium (100Fm) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in fallout from nuclear testing) was 255Fm in 1952. 250Fm was independently synthesized shortly after the discovery of 255Fm. There are 20 known radioisotopes ranging in atomic mass from 241Fm to 260Fm (260Fm is unconfirmed), and 2 nuclear isomers, 250mFm and 251mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 250mFm with a half-life of 1.8 seconds.

Isotopes of helium

Although there are nine known isotopes of helium (2He) (standard atomic weight: 4.002602(2)), only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 806.7 milliseconds. The least stable is 5He, with a half-life of 7.6×10−22 s, although it is possible that 2He has an even shorter half-life.

In the Earth's atmosphere, there is one 3He atom for every million 4He atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3He is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid 3He and 4He below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: 4He atoms are bosons while 3He atoms are fermions). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

Isotopes of mendelevium

Mendelevium (101Md) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was 256Md (which was also the first isotope of any element produced one atom at a time) in 1955. There are 16 known radioisotopes, ranging in atomic mass from 245Md to 260Md, and 5 isomers. The longest-lived isotope is 258Md with a half-life of 51.3 days, and the longest-lived isomer is 258mMd with a half-life of 57 minutes.

Isotopes of moscovium

Moscovium (115Mc) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 288Mc in 2004. There are four known radioisotopes from 287Mc to 290Mc. The longest-lived isotope is 290Mc with a half-life of 0.8 seconds.

Isotopes of oganesson

Oganesson (118Og) is a synthetic element created in particle accelerators, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first (and so far only) isotope to be synthesized was 294Og in 2002 and 2005; it has a half-life of 0.7 milliseconds. An unconfirmed isotope, 295Og, may have been observed in 2011 with a longer half-life of 181 milliseconds.

Isotopes of radium

Radium (88Ra) has no stable or nearly stable isotopes, and thus a standard atomic weight cannot be given. The longest lived, and most common, isotope of radium is 226Ra with a half-life of 1,600 years. 226Ra occurs in the decay chain of 238U (often referred to as the radium series.) Radium has 33 known isotopes from 202Ra to 234Ra.

In 2013 it was discovered that the nucleus of radium-224 is pear-shaped. This was the first discovery of an asymmetric nucleus.

Isotopes of sodium

There are 21 recognized isotopes of sodium (11Na), ranging from 18Na to 39Na and two isomers (22mNa and 24mNa). 23Na is the only stable (and the only primordial) isotope. As such, it is considered a monoisotopic element and it has a standard atomic weight of 22.98976928(2). Sodium has two radioactive cosmogenic isotopes (22Na, half-life = 2.605 years; and 24Na, half-life ≈ 15 hours). With the exception of those two, all other isotopes have half-lives under a minute, most under a second. The shortest-lived is 18Na, with a half-life of 1.3(4)×10−21 seconds.

Acute neutron radiation exposure (e.g., from a nuclear criticality accident) converts some of the stable 23Na in human blood plasma to 24Na. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed.

22Na is a positron-emitting isotope with a remarkably long half-life. It is used to create test-objects and point-sources for positron emission tomography.

Isotopes of tennessine

Tennessine (117Ts) is the most-recently synthesized synthetic element, and much of the data is hypothetical. As for any synthetic element, a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first (and so far only) isotopes to be synthesized were 293Ts and 294Ts in 2009. The longer-lived isotope is 294Ts with a half-life of 51 ms.

Periodic table (detailed cells)

The periodic table is a tabular method of displaying the chemical elements. It can show much information, after name, symbol and atomic number. Also, for each element mean atomic mass value for the natural isotopic composition of each element can be noted.

The two layout forms originate from two graphic forms of presentation of the same periodic table. Historically, when the f-block was identified it was drawn below the existing table, with markings for its in-table location (this page uses dots or asterisks). Also, a common presentation is to put all 15 lanthanide and actinide columns below, while the f-block only has 14 columns. One lanthanide and actinide each are d-block elements, belonging to group 3 with scandium and yttrium, though whether these are the first of each series (lanthanum and actinium) or the last (lutetium and lawrencium) has been disputed. The tables below show lanthanum and actinium as group 3 elements, as this is the more common form in the literature.

Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869. Mendeleev invented the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

Relative atomic mass

Relative atomic mass (symbol: Ar) or atomic weight is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to one unified atomic mass unit. The unified atomic mass unit (symbol: u or Da) is defined as being ​1⁄12 of the atomic mass of a carbon-12 atom. Since both values in the ratio are expressed in the same unit (u), the resulting value is dimensionless; hence the value is said to be relative.

For a single given sample, the relative atomic mass of a given element is the weighted arithmetic mean of the masses of the individual atoms (including their isotopes) that are present in the sample. This quantity can vary substantially between samples because the sample's origin (and therefore its radioactive history or diffusion history) may have produced unique combinations of isotopic abundances. For example, due to a different mixture of stable carbon-12 and carbon-13 isotopes, a sample of elemental carbon from volcanic methane will have a different relative atomic mass than one collected from plant or animal tissues.

The more common, and more specific quantity known as standard atomic weight (Ar, standard) is an application of the relative atomic mass values obtained from multiple different samples. It is sometimes interpreted as the expected range of the relative atomic mass values for the atoms of a given element from all terrestrial sources, with the various sources being taken from Earth. "Atomic weight" is often loosely and incorrectly used as a synonym for standard atomic weight (incorrectly because standard atomic weights are not from a single sample). Standard atomic weight is nevertheless the most widely published variant of relative atomic mass.

Additionally, the continued use of the term "atomic weight" (for any element) as opposed to "relative atomic mass" has attracted considerable controversy since at least the 1960s, mainly due to the technical difference between weight and mass in physics. Still, both terms are officially sanctioned by the IUPAC. The term "relative atomic mass" now seems to be replacing "atomic weight" as the preferred term, although the term "standard atomic weight" (as opposed to the more correct "standard relative atomic mass") continues to be used.

Transferability (chemistry)

Transferability, in chemistry, is the assumption that a chemical property that is associated with an atom or a functional group in a molecule will have a similar (but not identical) value in a variety of different circumstances. Examples of transferable properties include:



Chemical shifts in NMR spectroscopy

Characteristic frequencies in Infrared spectroscopy

Bond length and bond angle

Bond energyTransferable properties are distinguished from conserved properties, which are assumed to always have the same value whatever the chemical situation, e.g. standard atomic weight.

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