Atomic Mass of Every Element

IUPAC standard atomic weights for all 118 elements — the numbers to use for molar mass.

The masses below are the abridged standard atomic weights published by IUPAC's Commission on Isotopic Abundances and Atomic Weights — the same values printed on a classroom table, and the ones to use when you calculate molar mass. They include the 2024 revisions to gadolinium, lutetium and zirconium.

Two things on this table are not plain numbers, and both are honest rather than sloppy:

Bracketed values — 34 elements have no stable isotope, so no standard atomic weight exists. The bracketed figure is the mass number of the longest-lived isotope instead.
Intervals — 14 elements (H, Li, B, C, N, O, Mg, Si, S, Cl, Ar, Br, Tl, Pb) vary enough between natural sources that IUPAC publishes a range. The single value shown is the conventional one for calculations.

Standard atomic weight of every element
#	Element	Atomic mass (u)	IUPAC interval	Molar mass (g/mol)
1	HHydrogen	1.008	[1.00784, 1.00811]	1.008
2	HeHelium	4.0026	—	4.0026
3	LiLithium	6.94	[6.938, 6.997]	6.94
4	BeBeryllium	9.0122	—	9.0122
5	BBoron	10.81	[10.806, 10.821]	10.81
6	CCarbon	12.011	[12.0096, 12.0116]	12.011
7	NNitrogen	14.007	[14.00643, 14.00728]	14.007
8	OOxygen	15.999	[15.99903, 15.99977]	15.999
9	FFluorine	18.998	—	18.998
10	NeNeon	20.18	—	20.18
11	NaSodium	22.99	—	22.99
12	MgMagnesium	24.305	[24.304, 24.307]	24.305
13	AlAluminum	26.982	—	26.982
14	SiSilicon	28.085	[28.084, 28.086]	28.085
15	PPhosphorus	30.974	—	30.974
16	SSulfur	32.06	[32.059, 32.076]	32.06
17	ClChlorine	35.45	[35.446, 35.457]	35.45
18	ArArgon	39.95	[39.792, 39.963]	39.95
19	KPotassium	39.098	—	39.098
20	CaCalcium	40.078	—	40.078
21	ScScandium	44.956	—	44.956
22	TiTitanium	47.867	—	47.867
23	VVanadium	50.942	—	50.942
24	CrChromium	51.996	—	51.996
25	MnManganese	54.938	—	54.938
26	FeIron	55.845	—	55.845
27	CoCobalt	58.933	—	58.933
28	NiNickel	58.693	—	58.693
29	CuCopper	63.546	—	63.546
30	ZnZinc	65.38	—	65.38
31	GaGallium	69.723	—	69.723
32	GeGermanium	72.63	—	72.63
33	AsArsenic	74.922	—	74.922
34	SeSelenium	78.971	—	78.971
35	BrBromine	79.904	[79.901, 79.907]	79.904
36	KrKrypton	83.798	—	83.798
37	RbRubidium	85.468	—	85.468
38	SrStrontium	87.62	—	87.62
39	YYttrium	88.906	—	88.906
40	ZrZirconium	91.222	—	91.222
41	NbNiobium	92.906	—	92.906
42	MoMolybdenum	95.95	—	95.95
43	TcTechnetium	[98]	—	98
44	RuRuthenium	101.07	—	101.07
45	RhRhodium	102.91	—	102.91
46	PdPalladium	106.42	—	106.42
47	AgSilver	107.87	—	107.87
48	CdCadmium	112.41	—	112.41
49	InIndium	114.82	—	114.82
50	SnTin	118.71	—	118.71
51	SbAntimony	121.76	—	121.76
52	TeTellurium	127.6	—	127.6
53	IIodine	126.9	—	126.9
54	XeXenon	131.29	—	131.29
55	CsCesium	132.91	—	132.91
56	BaBarium	137.33	—	137.33
57	LaLanthanum	138.91	—	138.91
58	CeCerium	140.12	—	140.12
59	PrPraseodymium	140.91	—	140.91
60	NdNeodymium	144.24	—	144.24
61	PmPromethium	[145]	—	145
62	SmSamarium	150.36	—	150.36
63	EuEuropium	151.96	—	151.96
64	GdGadolinium	157.25	—	157.25
65	TbTerbium	158.93	—	158.93
66	DyDysprosium	162.5	—	162.5
67	HoHolmium	164.93	—	164.93
68	ErErbium	167.26	—	167.26
69	TmThulium	168.93	—	168.93
70	YbYtterbium	173.05	—	173.05
71	LuLutetium	174.97	—	174.97
72	HfHafnium	178.49	—	178.49
73	TaTantalum	180.95	—	180.95
74	WTungsten	183.84	—	183.84
75	ReRhenium	186.21	—	186.21
76	OsOsmium	190.23	—	190.23
77	IrIridium	192.22	—	192.22
78	PtPlatinum	195.08	—	195.08
79	AuGold	196.97	—	196.97
80	HgMercury	200.59	—	200.59
81	TlThallium	204.38	[204.382, 204.385]	204.38
82	PbLead	207.2	[206.14, 207.94]	207.2
83	BiBismuth	208.98	—	208.98
84	PoPolonium	[209]	—	209
85	AtAstatine	[210]	—	210
86	RnRadon	[222]	—	222
87	FrFrancium	[223]	—	223
88	RaRadium	[226]	—	226
89	AcActinium	[227]	—	227
90	ThThorium	232.04	—	232.04
91	PaProtactinium	231.04	—	231.04
92	UUranium	238.03	—	238.03
93	NpNeptunium	[237]	—	237
94	PuPlutonium	[244]	—	244
95	AmAmericium	[243]	—	243
96	CmCurium	[247]	—	247
97	BkBerkelium	[247]	—	247
98	CfCalifornium	[251]	—	251
99	EsEinsteinium	[252]	—	252
100	FmFermium	[257]	—	257
101	MdMendelevium	[258]	—	258
102	NoNobelium	[259]	—	259
103	LrLawrencium	[266]	—	266
104	RfRutherfordium	[267]	—	267
105	DbDubnium	[268]	—	268
106	SgSeaborgium	[269]	—	269
107	BhBohrium	[270]	—	270
108	HsHassium	[269]	—	269
109	MtMeitnerium	[278]	—	278
110	DsDarmstadtium	[281]	—	281
111	RgRoentgenium	[282]	—	282
112	CnCopernicium	[285]	—	285
113	NhNihonium	[286]	—	286
114	FlFlerovium	[289]	—	289
115	McMoscovium	[290]	—	290
116	LvLivermorium	[293]	—	293
117	TsTennessine	[294]	—	294
118	OgOganesson	[294]	—	294

Why atomic masses are not whole numbers

A single atom's mass is very nearly a whole number of nucleons. But an element is a mixture of isotopes, and the table shows the weighted average of that mixture. Chlorine reads 35.45 because natural chlorine is roughly 76% chlorine-35 and 24% chlorine-37 — no individual chlorine atom weighs 35.45 u, but a mole of natural chlorine behaves as though every atom did.

The other reason is subtler: a nucleus weighs slightly less than its parts, because binding energy is mass that has been released. That defect is why fusion and fission release energy at all, and it is why even carbon-12 is exactly 12 only by definition.

More on the periodic table

Interactive periodic tableAll 118 elements Charges & oxidation statesWhat ion each element forms Electron configurationsFull and shorthand notation ElectronegativityPauling values, every element Valence electronsOuter-shell counts by group Groups & periodsAll 18 groups explained Printable periodic tablesFree PDFs to print Element quizTest yourself on symbols

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Frequently Asked Questions

What is atomic mass?

Atomic mass is the mass of an atom in unified atomic mass units (u), where 1 u is defined as one twelfth of the mass of a carbon-12 atom. The value printed on a periodic table is the standard atomic weight: an average across the element's isotopes, weighted by how common each one is in nature.

Is atomic mass the same as molar mass?

Numerically yes, conceptually no. An element's atomic mass in u equals its molar mass in grams per mole, which is what makes the periodic table so useful in the lab: iron reads 55.845, so one mole of iron atoms weighs 55.845 g. Atomic mass describes one atom; molar mass describes 6.022 × 10²³ of them.

Why are some atomic masses shown in square brackets?

Because those elements have no stable isotope and no consistent natural isotopic composition, so a standard atomic weight cannot be defined for them. The bracketed number is instead the mass number of the longest-lived isotope. Thirty-four elements on the table are in this position, including technetium, promethium and everything from polonium upward.

Why does IUPAC give some atomic masses as a range?

For fourteen elements the isotopic mix varies measurably depending on where the sample came from, so a single number would be misleading. Hydrogen is published as [1.00784, 1.00811] and carbon as [12.0096, 12.0116]. For ordinary calculations use the conventional single value; the interval matters when the source of the material matters, as in isotope geochemistry.

How do I calculate the molar mass of a compound?

Add up the atomic mass of every atom in the formula. Water, H₂O, is 2 × 1.008 + 15.999 ≈ 18.02 g/mol. Subscripts multiply the element immediately before them, and anything in brackets is multiplied through. Our molecular weight calculator does this for any formula you type.