Atomic Weight Reference Table — Free Chemistry Reference

Complete IUPAC 2021 standard atomic weights for all 118 elements — sortable, searchable, filterable by element category.

Showing 118 of 118 elements

			Category
1	H	1.008	Hydrogen	Nonmetal
2	He	4.003	Helium	Noble Gas
3	Li	6.940	Lithium	Alkali Metal
4	Be	9.012	Beryllium	Alkaline Earth
5	B	10.810	Boron	Metalloid
6	C	12.011	Carbon	Nonmetal
7	N	14.007	Nitrogen	Nonmetal
8	O	15.999	Oxygen	Nonmetal
9	F	18.998	Fluorine	Halogen
10	Ne	20.180	Neon	Noble Gas
11	Na	22.990	Sodium	Alkali Metal
12	Mg	24.305	Magnesium	Alkaline Earth
13	Al	26.982	Aluminium	Post-Transition Metal
14	Si	28.085	Silicon	Metalloid
15	P	30.974	Phosphorus	Nonmetal
16	S	32.060	Sulfur	Nonmetal
17	Cl	35.450	Chlorine	Halogen
18	Ar	39.948	Argon	Noble Gas
19	K	39.098	Potassium	Alkali Metal
20	Ca	40.078	Calcium	Alkaline Earth
21	Sc	44.956	Scandium	Transition Metal
22	Ti	47.867	Titanium	Transition Metal
23	V	50.942	Vanadium	Transition Metal
24	Cr	51.996	Chromium	Transition Metal
25	Mn	54.938	Manganese	Transition Metal
26	Fe	55.845	Iron	Transition Metal
27	Co	58.933	Cobalt	Transition Metal
28	Ni	58.693	Nickel	Transition Metal
29	Cu	63.546	Copper	Transition Metal
30	Zn	65.380	Zinc	Transition Metal
31	Ga	69.723	Gallium	Post-Transition Metal
32	Ge	72.630	Germanium	Metalloid
33	As	74.922	Arsenic	Metalloid
34	Se	78.971	Selenium	Nonmetal
35	Br	79.904	Bromine	Halogen
36	Kr	83.798	Krypton	Noble Gas
37	Rb	85.468	Rubidium	Alkali Metal
38	Sr	87.620	Strontium	Alkaline Earth
39	Y	88.906	Yttrium	Transition Metal
40	Zr	91.224	Zirconium	Transition Metal
41	Nb	92.906	Niobium	Transition Metal
42	Mo	95.950	Molybdenum	Transition Metal
43*	Tc	*97.000	Technetium	Transition Metal
44	Ru	101.070	Ruthenium	Transition Metal
45	Rh	102.906	Rhodium	Transition Metal
46	Pd	106.420	Palladium	Transition Metal
47	Ag	107.868	Silver	Transition Metal
48	Cd	112.414	Cadmium	Transition Metal
49	In	114.818	Indium	Post-Transition Metal
50	Sn	118.710	Tin	Post-Transition Metal
51	Sb	121.760	Antimony	Metalloid
52	Te	127.600	Tellurium	Metalloid
53	I	126.904	Iodine	Halogen
54	Xe	131.293	Xenon	Noble Gas
55	Cs	132.905	Caesium	Alkali Metal
56	Ba	137.327	Barium	Alkaline Earth
57	La	138.905	Lanthanum	Lanthanide
58	Ce	140.116	Cerium	Lanthanide
59	Pr	140.908	Praseodymium	Lanthanide
60	Nd	144.242	Neodymium	Lanthanide
61*	Pm	*145.000	Promethium	Lanthanide
62	Sm	150.360	Samarium	Lanthanide
63	Eu	151.964	Europium	Lanthanide
64	Gd	157.250	Gadolinium	Lanthanide
65	Tb	158.925	Terbium	Lanthanide
66	Dy	162.500	Dysprosium	Lanthanide
67	Ho	164.930	Holmium	Lanthanide
68	Er	167.259	Erbium	Lanthanide
69	Tm	168.934	Thulium	Lanthanide
70	Yb	173.045	Ytterbium	Lanthanide
71	Lu	174.967	Lutetium	Lanthanide
72	Hf	178.490	Hafnium	Transition Metal
73	Ta	180.948	Tantalum	Transition Metal
74	W	183.840	Tungsten	Transition Metal
75	Re	186.207	Rhenium	Transition Metal
76	Os	190.230	Osmium	Transition Metal
77	Ir	192.217	Iridium	Transition Metal
78	Pt	195.084	Platinum	Transition Metal
79	Au	196.967	Gold	Transition Metal
80	Hg	200.592	Mercury	Transition Metal
81	Tl	204.380	Thallium	Post-Transition Metal
82	Pb	207.200	Lead	Post-Transition Metal
83	Bi	208.980	Bismuth	Post-Transition Metal
84*	Po	*209.000	Polonium	Post-Transition Metal
85*	At	*210.000	Astatine	Halogen
86*	Rn	*222.000	Radon	Noble Gas
87*	Fr	*223.000	Francium	Alkali Metal
88*	Ra	*226.000	Radium	Alkaline Earth
89*	Ac	*227.000	Actinium	Actinide
90	Th	232.038	Thorium	Actinide
91	Pa	231.036	Protactinium	Actinide
92	U	238.029	Uranium	Actinide
93*	Np	*237.000	Neptunium	Actinide
94*	Pu	*244.000	Plutonium	Actinide
95*	Am	*243.000	Americium	Actinide
96*	Cm	*247.000	Curium	Actinide
97*	Bk	*247.000	Berkelium	Actinide
98*	Cf	*251.000	Californium	Actinide
99*	Es	*252.000	Einsteinium	Actinide
100*	Fm	*257.000	Fermium	Actinide
101*	Md	*258.000	Mendelevium	Actinide
102*	No	*259.000	Nobelium	Actinide
103*	Lr	*262.000	Lawrencium	Actinide
104*	Rf	*267.000	Rutherfordium	Transition Metal
105*	Db	*270.000	Dubnium	Transition Metal
106*	Sg	*271.000	Seaborgium	Transition Metal
107*	Bh	*270.000	Bohrium	Transition Metal
108*	Hs	*277.000	Hassium	Transition Metal
109*	Mt	*276.000	Meitnerium	Transition Metal
110*	Ds	*281.000	Darmstadtium	Transition Metal
111*	Rg	*282.000	Roentgenium	Transition Metal
112*	Cn	*285.000	Copernicium	Transition Metal
113*	Nh	*286.000	Nihonium	Post-Transition Metal
114*	Fl	*289.000	Flerovium	Post-Transition Metal
115*	Mc	*290.000	Moscovium	Post-Transition Metal
116*	Lv	*293.000	Livermorium	Post-Transition Metal
117*	Ts	*294.000	Tennessine	Halogen
118*	Og	*294.000	Oganesson	Noble Gas

About Atomic Weights

What Is Atomic Weight?

Atomic weight (relative atomic mass) is the weighted average mass of all naturally occurring isotopes of an element, expressed relative to 1/12 the mass of carbon-12. It accounts for natural isotopic abundance and is the value used in chemical stoichiometry.

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IUPAC 2021 Standard Values

The International Union of Pure and Applied Chemistry (IUPAC) publishes standard atomic weights through the Commission on Isotopic Abundances and Atomic Weights (CIAAW). These values are updated as isotope measurements improve. The 2021 revision is the current reference.

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NIST Cross-Reference

The National Institute of Standards and Technology (NIST) maintains the Atomic Weights and Isotopic Compositions database (SRD 144). All values in this table are verified against both IUPAC CIAAW and NIST SRD 144 for accuracy.

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Elements Without Stable Isotopes

Thirty-eight elements have no stable isotopes (marked with *). For these, the atomic weight shown is the mass number of the most stable known isotope — the convention adopted by IUPAC for elements with no natural terrestrial abundance.

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Frequently Asked Questions

What is the difference between atomic weight and mass number?

Mass number is the total number of protons and neutrons in a single isotope (always a whole number, e.g. carbon-12 = 12). Atomic weight is the weighted average of all naturally occurring isotopes of an element (e.g. carbon = 12.011), reflecting isotopic abundance on Earth.

Why do some atomic weights have only a few decimal places while others have six?

Atomic weight precision depends on the variability of isotope abundance measurements. Elements with one dominant isotope (like fluorine-19 at nearly 100%) have very precise values. Elements with widely variable isotope ratios in nature (like lead) have wider uncertainty ranges.

Which elements have no stable isotopes — and why are they included?

Technetium (43), Promethium (61), and all elements with atomic number 84 and above have no stable isotopes. They are included using the mass of their most stable isotope per IUPAC convention, marked with an asterisk (*) in the table.

How often are atomic weight values updated?

IUPAC publishes biennial updates to standard atomic weights. Major revisions (like the 2021 update that introduced interval notation for 12 elements) occur when new isotope measurements change the accepted value. The core data in this table reflects the 2021 standard.

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About Atomic Weights

What Is Atomic Weight?

IUPAC 2021 Standard Values

NIST Cross-Reference

Elements Without Stable Isotopes

Frequently Asked Questions

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