Atomic weight

Not to be confused with atomic mass.

Atomic weight (symbol: A_r) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element (from a given source) to 1/12 of the mass of an atom of carbon-12 (known as the unified atomic mass unit).^[1]^[2] The term is usually used, without further qualification, to refer to the standard atomic weights published at regular intervals by the International Union of Pure and Applied Chemistry (IUPAC)^[3]^[4] and which are intended to be applicable to normal laboratory materials. These standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, wallcharts etc., and in the table below.

The term "relative atomic mass" (of the element) may also be used to describe this physical quantity, and is synonymous with it. Indeed the continued use of the term "atomic weight" has attracted considerable controversy since at least the 1960s^[5] (see below).

Atomic weights, unlike atomic masses (the masses of individual atoms, not to be confused with relative atomic mass), are not physical constants, but vary from sample to sample of elements that are not mononuclidic elements. This is due to differing isotopic distributions in various samples of non-mononuclidics. Nevertheless, even for elements naturally consisting of two or more nuclides, the atomic weights are sufficiently constant in "normal" samples (those drawn from the environment without special processing) to be of fundamental importance in chemistry.

1 Definition
2 Naming controversy
3 Determination of atomic weight
4 Periodic table by atomic weight
5 References
6 External links

Definition

The IUPAC definition^[1] of atomic weight is:

An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of ¹²C.

The definition deliberately specifies "An atomic weight…", as an element will have different atomic weights depending on the source. For example, boron from Turkey has a lower atomic weight than boron from California, because of its different isotopic composition.^[6]^[7] Nevertheless, given the cost and difficulty of isotope analysis, it is usual to use the tabulated values of standard atomic weights which are ubiquitous in chemical laboratories.

Naming controversy

The use of the name "atomic weight" has attracted a great deal of controversy among scientists.^[5] 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.

In reply, supporters of the term "atomic weight" point out (among other arguments)^[5] that

the name has been in continuous use for the same quantity since it was first conceptualized in 1808;^[8]
for most of that time, atomic weights really were measured by weighing (that is by gravimetric analysis) and that 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
- electromotive force, which is not a force
- resolving power, which is not a power quantity
- molar concentration, which is not a molar quantity (a quantity expressed per unit amount of substance)

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.

Determination of atomic weight

Modern atomic weights are calculated from measured values of atomic mass (for each nuclide) and isotopic composition. Highly accurate atomic masses are available^[9]^[10] for virtually all non-radioactive nuclides, but isotopic compositions are both harder to measure to high precision and more subject to variation between samples.^[11]^[12] For this reason, the atomic weights of the twenty-two mononuclidic elements are known to especially high accuracy – an uncertainty of only one part in 38 million in the case of fluorine, a precision which is greater than the current best value for the Avogadro constant (one part in 20 million).

Isotope	Atomic mass^[10]	Abundance^[11]
Isotope	Atomic mass^[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)%	4.69–4.67%
³⁰Si	29.973 770 171(32)	3.0872(5)%	3.10–3.08%

The calculation is exemplified for silicon, whose atomic weight 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

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

The estimation of the uncertainty is complicated,^[13] especially as the sample distribution is not necessarily symmetrical: the IUPAC standard atomic weights are quoted with estimated symmetrical uncertainties,^[14] and the value for silicon is 28.0855(3). The relative standard uncertainty in this value is 1×10^–5 or 10 ppm.

Periodic table by atomic weight

Group →	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	H 1.008																	He 4.003
2	Li 6.941	Be 9.012											B 10.81	C 12.01	N 14.01	O 16.00	F 19.00	Ne 20.18
3	Na 22.99	Mg 24.31											Al 26.98	Si 28.09	P 30.97	S 32.07	Cl 35.45	Ar 39.95
4	K 39.10	Ca 40.08	Sc 44.96	Ti 47.87	V 50.94	Cr 52.00	Mn 54.94	Fe 55.84	Co 58.93	Ni 58.69	Cu 63.55	Zn 65.39	Ga 69.72	Ge 72.63	As 74.92	Se 78.96	Br 79.90	Kr 83.80
5	Rb 85.47	Sr 87.62	Y 88.91	Zr 91.22	Nb 92.91	Mo 95.94	Tc [98]	Ru 101.07	Rh 102.91	Pd 106.42	Ag 107.87	Cd 112.41	In 114.82	Sn 118.71	Sb 121.76	Te 127.60	I 126.90	Xe 131.29
6	Cs 132.91	Ba 137.33	*	Hf 178.49	Ta 180.95	W 183.84	Re 186.21	Os 190.23	Ir 192.22	Pt 195.08	Au 196.97	Hg 200.59	Tl 204.38	Pb 207.2	Bi 208.98	Po [209]	At [210]	Rn [222]
7	Fr [223]	Ra [226]	**	Rf [267]	Db [268]	Sg [269]	Bh [270]	Hs [269]	Mt [278]	Ds [281]	Rg [281]	Cn [283]	Uut [286]	Uuq [289]	Uup [289]	Uuh [293]	Uus [294]	Uuo [294]

* Lanthanoids				La 138.91	Ce 140.12	Pr 140.91	Nd 144.24	Pm [145]	Sm 150.36	Eu 151.96	Gd 157.25	Tb 158.93	Dy 162.50	Ho 164.93	Er 167.26	Tm 168.93	Yb 173.04	Lu 174.97
Actinoids**				Ac [227]	Th 232.04	Pa 231.04	U 238.03	Np [237]	Pu [244]	Am [243]	Cm [247]	Bk [247]	Cf [251]	Es [252]	Fm [257]	Md [258]	No [259]	Lr [262]

Element categories in the periodic table

Metals						Metalloids	Nonmetals			Unknown chemical properties
Alkali metals	Alkaline earth metals	Inner transition metals		Transition metals	Post-transition metals		Other nonmetals	Halogens	Noble gases
		Lanthanides	Actinides

**Atomic number colors show state of matter at standard conditions (0 °C and 1 atm):**
Solids	Liquids	Gases	Unknown

**Borders show natural occurrence:**
Primordial	From decay	Synthetic

References

^ ^a ^b International Union of Pure and Applied Chemistry (1980). "Atomic Weights of the Elements 1979". Pure Appl. Chem. 52 (10): 2349–84. doi:10.1351/pac198052102349. http://www.iupac.org/publications/pac/1980/pdf/5210x2349.pdf.
^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 41. Electronic version.
^ The latest edition is International Union of Pure and Applied Chemistry (2006). "Atomic Weights of the Elements 2005". Pure Appl. Chem. 78 (11): 2051–66. doi:10.1351/pac200678112051. http://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf.
^ The updated list of standard atomic weights is expected to be formally published in late 2008. The IUPAC(International Union Of Pure And Applied Chemistry) Commission on Isotopic Abundances and Atomic Weights announced in August 2007 that the standard atomic weights of the following elements would be revised (new figures quoted here): lutetium 174.9668(1); molybdenum 95.96(2); nickel 58.6934(4); ytterbium 173.054(5); zinc 65.38(2). The recommended value for the isotope amount ratio of ⁴⁰Ar/³⁶Ar (which could be useful as a control measurement in argon–argon dating) was also changed from 296.03(53) to 298.56(31).
^ ^a ^b ^c 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. http://www.iupac.org/publications/pac/1992/pdf/6410x1535.pdf.
^ Greenwood, Norman N.; Earnshaw, A. (1984). Chemistry of the Elements. Oxford: Pergamon. pp. pp. 21, 160. ISBN 0-08-022057-6.
^ International Union of Pure and Applied Chemistry (2003). "Atomic Weights of the Elements: Review 2000". Pure Appl. Chem. 75 (6): 683–800. doi:10.1351/pac200375060683. http://www.iupac.org/publications/pac/2003/pdf/7506x0683.pdf.
^ Dalton, John (1808). A New System of Chemical Philosophy. Manchester. http://www.archive.org/details/newsystemofchemi01daltuoft.
^ National Institute of Standards and Technology. Atomic Weights and Isotopic Compositions for All Elements.
^ ^a ^b Wapstra, A.H.; Audi, G.; Thibault, C. (2003), "The AME2003 atomic mass evaluation (I). Evaluation of input data, adjustment procedures", Nucl. Phys. A 729: 129–336, Bibcode 2003NuPhA.729..129W, doi:10.1016/j.nuclphysa.2003.11.002 Wapstra, A.H.; Audi, G.; Thibault, C. (2003), "The AME2003 atomic mass evaluation (II). Tables, graphs, and references", Nucl. Phys. A 729: 337–676, Bibcode 2003NuPhA.729..337A, doi:10.1016/j.nuclphysa.2003.11.003 Data tables.
^ ^a ^b International Union of Pure and Applied Chemistry (1998), "Isotopic Compositions of the Elements 1997", Pure Appl. Chem. 70 (1): 217–35, doi:10.1351/pac199870010217, http://media.iupac.org/publications/pac/1998/pdf/7001x0217.pdf
^ International Union of Pure and Applied Chemistry (2002), "Isotopic Abundance Variations Of Selected Elements", Pure Appl. Chem. 74 (10): 1987–2017, http://www.iupac.org/publications/pac/2002/pdf/7410x1987.pdf
^ Meija, Juris; Mester, Zoltán (2008). "Uncertainty propagation of atomic weight measurement results". Metrologia 45: 53–62. doi:10.1088/0026-1394/45/1/008. http://stacks.iop.org/Met/45/53.
^ Holden, Norman E. (2004). "Atomic Weights and the International Committee—A Historical Review". Chemistry International 26 (1): 4–7. http://www.iupac.org/publications/ci/2004/2601/1_holden.html.

External links

IUPAC Commission on Isotopic Abundances and Atomic Weights
NIST relative atomic masses of all isotopes and the standard atomic weights of the elements
Atomic Weights and the International Committee — A Historical Review
Atomic Weights of the Elements 2009 – semi-official compilation in advance of the formal publication of the report

Periodic tables

Layouts	Standard Large table Inline f-block Vertical Text only Metals and nonmetals Blocks Valences Extension beyond the 7th period Large extended table Electron configurations Electronegativities Alternatives Crystal structure Discovery periods

List of elements by	Name etymology (symbol) Discovery Oxidation state Abundance (in humans) Nuclear stability Hardness Atomic properties Production

Groups	1 (Alkali metals) 2 (Alkaline earth metals) 3 4 5 6 7 8 9 10 11 12 13 (Boron group) 14 (Carbon group) 15 (Pnictogens) 16 (Chalcogens) 17 (Halogens) 18 (Noble gases)

Other element categories	Periods Metals Transition metals Metalloids Nonmetals Lanthanides Actinides Rare earth elements Platinum group metals (PGMs) Post-transition metals Refractory metals

Blocks	s-block p-block d-block f-block g-block

Periods	1 2 3 4 5 6 7 8

Book:Periodic table Category:Periodic table Portal:Chemistry