Standard Atomic Weights | Commission on Isotopic Abundances and Atomic Weights

Standard atomic weights are CIAAW recommended values for atomic weights applicable to all normal materials. Since 1902, the Commission regularly publishes critical evaluation of atomic weights of elements and below is the most recent definitive table of the standard atomic weights.

Standard Atomic Weights 2024

Z	Symbol	Element	Standard Atomic Weight	Notes
1	H	hydrogen	[1.007 84, 1.008 11]	m
2	He	helium	4.002 602(2)	g r
3	Li	lithium	[6.938, 6.997]	m
4	Be	beryllium	9.012 1831(5)
5	B	boron	[10.806, 10.821]	m
6	C	carbon	[12.0096, 12.0116]
7	N	nitrogen	[14.006 43, 14.007 28]	m
8	O	oxygen	[15.999 03, 15.999 77]	m
9	F	fluorine	18.998 403 162(5)
10	Ne	neon	20.1797(6)	g m
11	Na	sodium	22.989 769 28(2)
12	Mg	magnesium	[24.304, 24.307]
13	Al	aluminium	26.981 5384(3)
14	Si	silicon	[28.084, 28.086]
15	P	phosphorus	30.973 761 998(5)
16	S	sulfur	[32.059, 32.076]
17	Cl	chlorine	[35.446, 35.457]	m
18	Ar	argon	[39.792, 39.963]
19	K	potassium	39.0983(1)
20	Ca	calcium	40.078(4)	g
21	Sc	scandium	44.955 907(4)
22	Ti	titanium	47.867(1)
23	V	vanadium	50.9415(1)
24	Cr	chromium	51.9961(6)
25	Mn	manganese	54.938 043(2)
26	Fe	iron	55.845(2)
27	Co	cobalt	58.933 194(3)
28	Ni	nickel	58.6934(4)	r
29	Cu	copper	63.546(3)	r
30	Zn	zinc	65.38(2)	r
31	Ga	gallium	69.723(1)
32	Ge	germanium	72.630(8)
33	As	arsenic	74.921 595(6)
34	Se	selenium	78.971(8)	r
35	Br	bromine	[79.901, 79.907]
36	Kr	krypton	83.798(2)	g m
37	Rb	rubidium	85.4678(3)	g
38	Sr	strontium	87.62(1)	g r
39	Y	yttrium	88.905 838(2)
40	Zr	zirconium	91.222(3)	g
41	Nb	niobium	92.906 37(1)
42	Mo	molybdenum	95.95(1)	g
43	Tc	technetium	—
44	Ru	ruthenium	101.07(2)	g
45	Rh	rhodium	102.905 49(2)
46	Pd	palladium	106.42(1)	g
47	Ag	silver	107.8682(2)	g
48	Cd	cadmium	112.414(4)	g
49	In	indium	114.818(1)
50	Sn	tin	118.710(7)	g
51	Sb	antimony	121.760(1)	g
52	Te	tellurium	127.60(3)	g
53	I	iodine	126.904 47(3)
54	Xe	xenon	131.293(6)	g m
55	Cs	caesium	132.905 451 96(6)
56	Ba	barium	137.327(7)
57	La	lanthanum	138.905 47(7)	g
58	Ce	cerium	140.116(1)	g
59	Pr	praseodymium	140.907 66(1)
60	Nd	neodymium	144.242(3)	g
61	Pm	promethium	—
62	Sm	samarium	150.36(2)	g
63	Eu	europium	151.964(1)	g
64	Gd	gadolinium	157.249(2)	g
65	Tb	terbium	158.925 354(7)
66	Dy	dysprosium	162.500(1)	g
67	Ho	holmium	164.930 329(5)
68	Er	erbium	167.259(3)	g
69	Tm	thulium	168.934 219(5)
70	Yb	ytterbium	173.045(10)	g
71	Lu	lutetium	174.966 69(5)	g
72	Hf	hafnium	178.486(6)	g
73	Ta	tantalum	180.947 88(2)
74	W	tungsten	183.84(1)
75	Re	rhenium	186.207(1)
76	Os	osmium	190.23(3)	g
77	Ir	iridium	192.217(2)
78	Pt	platinum	195.084(9)
79	Au	gold	196.966 570(4)
80	Hg	mercury	200.592(3)
81	Tl	thallium	[204.382, 204.385]
82	Pb	lead	[206.14, 207.94]
83	Bi	bismuth	208.980 40(1)
84	Po	polonium	—
85	At	astatine	—
86	Rn	radon	—
87	Fr	francium	—
88	Ra	radium	—
89	Ac	actinium	—
90	Th	thorium	232.0377(4)	g
91	Pa	protactinium	231.035 88(1)
92	U	uranium	238.028 91(3)	g m
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	—
Z	Symbol	Element	Standard Atomic Weight	Notes

Footnotes      Back to Top
g Geological and biological materials are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such materials and that given in the table may exceed the stated uncertainty.
m Modified isotopic compositions may be found in commercially available material because the material has been subjected to some undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the table can occur.
r Range in isotopic composition of normal terrestrial material prevents a more precise standard atomic weight being given; the tabulated value and uncertainty should be applicable to normal material.

Citation

The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2021" report. Standard Atomic Weights of gadolinium, lutetium, and zirconium have been revised by the CIAAW in 2024 and these revisions are included in this table.

There are three broad groups of elements depending on what is the main cause of the uncertainty of their standard atomic weights:
  (1) well-documented natural variations of isotopic abundances,
  (2) our ability to determine the isotopic abundances, and
  (3) our ability to precisely determine the atomic masses of the isotopes.
Elements in the first category are distinguished by an interval standard atomic weight.

The reported uncertainties of the standard atomic weights are such that the atomic-weight values of normal materials are expected to lie in the given interval with great certitude. Thus, the standard atomic weight of argon, A_r°(Ar) = [39.792, 39.963], indicates that atomic-weight values of argon in normal materials are expected to be from 39.792 to 39.963. For iridium, the standard atomic weight A_r°(Ir) = 192.217 ± 0.002 indicates that atomic-weight values of iridium in normal materials are expected to be from 192.215 to 192.219.
For more information on the interpretation of the uncertainty please consult the recent IUPAC Technical Report by Possolo et al.