Atomic Masses

This table lists selected isotopes. Numbers in parentheses give uncertainties in the last quoted digits; for example, 3.52(18) means 3.52 ± 0.18. Abundance ranges show natural variation. See the table notes for sources, decay-mode conventions, and metastable-state assignments.

Table A.1. Atomic Masses
Atomic Number, Z	Name	Atomic Mass Number, A	Symbol	Atomic Mass (u)	Natural Abundance / Principal Decay Mode(s)	Half-life, t_1/2
0	neutron	1	$n$	1.008665	$β^{-}$	609.8(6) s
1	Hydrogen	1	${}^{1}H$	1.007825	99.972–99.999%	Stable
	Deuterium	2	${}^{2}H$ or $D$	2.014102	0.001–0.028%	Stable
	Tritium	3	${}^{3}H$ or $T$	3.016049	$β^{-}$	12.32(2) y
2	Helium	3	${}^{3}{He}$	3.016029	0.0002(2)%	Stable
		4	${}^{4}{He}$	4.002603	99.9998(2)%	Stable
3	Lithium	6	${}^{6}{Li}$	6.015123	1.9–7.8%	Stable
		7	${}^{7}{Li}$	7.016003	92.2–98.1%	Stable
4	Beryllium	7	${}^{7}{Be}$	7.016929	EC	53.22(6) d
		9	${}^{9}{Be}$	9.012183	100%	Stable
5	Boron	10	${}^{10}B$	10.012937	18.9–20.4%	Stable
		11	${}^{11}B$	11.009305	79.6–81.1%	Stable
6	Carbon	11	${}^{11}C$	11.011433	EC/ $β^{+}$	20.3402(53) min
		12	${}^{12}C$	12.000000	98.84–99.04%	Stable
		13	${}^{13}C$	13.003355	0.96–1.16%	Stable
		14	${}^{14}C$	14.003242	$β^{-}$	$5.70(3) \times 10^{3} y$
7	Nitrogen	13	${}^{13}N$	13.005739	EC/ $β^{+}$	9.965(4) min
		14	${}^{14}N$	14.003074	99.578–99.663%	Stable
		15	${}^{15}N$	15.000109	0.337–0.422%	Stable
8	Oxygen	15	${}^{15}O$	15.003066(1)	EC/ $β^{+}$	122.266(43) s
		16	${}^{16}O$	15.994915	99.738–99.776%	Stable
		18	${}^{18}O$	17.999160	0.187–0.222%	Stable
9	Fluorine	18	${}^{18}F$	18.000937	EC/ $β^{+}$	109.734(8) min
		19	${}^{19}F$	18.998403	100%	Stable
10	Neon	20	${}^{20}{Ne}$	19.992440	90.48(3)%	Stable
		22	${}^{22}{Ne}$	21.991385	9.25(3)%	Stable
11	Sodium	22	${}^{22}{Na}$	21.994438	$β^{+}$ , EC	2.6019(6) y
		23	${}^{23}{Na}$	22.989769	100%	Stable
		24	${}^{24}{Na}$	23.990963	$β^{-}$	14.9560(15) h
12	Magnesium	24	${}^{24}{Mg}$	23.985042	78.88–79.05%	Stable
13	Aluminum	27	${}^{27}{Al}$	26.981538	100%	Stable
14	Silicon	28	${}^{28}{Si}$	27.976927	92.191–92.318%	Stable
		31	${}^{31}{Si}$	30.975363	$β^{-}$	157.16(20) min
15	Phosphorus	31	${}^{31}P$	30.973762	100%	Stable
		32	${}^{32}P$	31.973908	$β^{-}$	14.269(7) d
16	Sulfur	32	${}^{32}S$	31.972071	94.41–95.29%	Stable
		35	${}^{35}S$	34.969032	$β^{-}$	87.37(4) d
17	Chlorine	35	${}^{35}{Cl}$	34.968853	75.5–76.1%	Stable
		37	${}^{37}{Cl}$	36.965903	23.9–24.5%	Stable
18	Argon	40	${}^{40}{Ar}$	39.962383	93.6–100.0%	Stable
19	Potassium	39	${}^{39}K$	38.963706	93.2581(44)%	Stable
		40	${}^{40}K$	39.963998	0.0117(1)%, $β^{-}$ , EC/ $β^{+}$	$1.248(3) \times 10^{9} y$
20	Calcium	40	${}^{40}{Ca}$	39.962591	96.941(156)%	Stable
21	Scandium	45	${}^{45}{Sc}$	44.955907(1)	100%	Stable
22	Titanium	48	${}^{48}{Ti}$	47.947941	73.72(3)%	Stable
23	Vanadium	51	${}^{51}V$	50.943958	99.750(10)%	Stable
24	Chromium	52	${}^{52}{Cr}$	51.940505	83.789(18)%	Stable
25	Manganese	55	${}^{55}{Mn}$	54.938043	100%	Stable
26	Iron	56	${}^{56}{Fe}$	55.934936	91.754(106)%	Stable
27	Cobalt	59	${}^{59}{Co}$	58.933194	100%	Stable
		60	${}^{60}{Co}$	59.933816	$β^{-}$	5.2714(6) y
28	Nickel	58	${}^{58}{Ni}$	57.935342	68.0769(190)%	Stable
		60	${}^{60}{Ni}$	59.930785	26.2231(150)%	Stable
29	Copper	63	${}^{63}{Cu}$	62.929597	69.15(15)%	Stable
		65	${}^{65}{Cu}$	64.927789(1)	30.85(15)%	Stable
30	Zinc	64	${}^{64}{Zn}$	63.929142(1)	49.17(75)%	Stable
		66	${}^{66}{Zn}$	65.926034(1)	27.73(98)%	Stable
31	Gallium	69	${}^{69}{Ga}$	68.925574(1)	60.108(50)%	Stable
32	Germanium	72	${}^{72}{Ge}$	71.922076	27.45(15)%	Stable
		74	${}^{74}{Ge}$	73.921178	36.52(12)%	Stable
33	Arsenic	75	${}^{75}{As}$	74.921595(1)	100%	Stable
34	Selenium	80	${}^{80}{Se}$	79.916522(1)	49.80(36)%	Stable
35	Bromine	79	${}^{79}{Br}$	78.918338(1)	50.5–50.8%	Stable
36	Krypton	84	${}^{84}{Kr}$	83.911498	56.987(15)%	Stable
37	Rubidium	85	${}^{85}{Rb}$	84.911790	72.17(2)%	Stable
38	Strontium	86	${}^{86}{Sr}$	85.909261	9.86(20)%	Stable
		88	${}^{88}{Sr}$	87.905612	82.58(35)%	Stable
		90	${}^{90}{Sr}$	89.907728(2)	$β^{-}$	28.91(3) y
39	Yttrium	89	${}^{89}Y$	88.905838	100%	Stable
		90	${}^{90}Y$	89.907142	$β^{-}$	64.05(5) h
40	Zirconium	90	${}^{90}{Zr}$	89.904699	51.47(6)%	Stable
41	Niobium	93	${}^{93}{Nb}$	92.906373(2)	100%	Stable
42	Molybdenum	98	${}^{98}{Mo}$	97.905404	24.292(80)%	Stable
43	Technetium	98	${}^{98}{Tc}$	97.907211(4)	$β^{-}$	$4.2(3) \times 10^{6} y$
44	Ruthenium	102	${}^{102}{Ru}$	101.904340	31.55(14)%	Stable
45	Rhodium	103	${}^{103}{Rh}$	102.905494(2)	100%	Stable
46	Palladium	106	${}^{106}{Pd}$	105.903480(1)	27.33(3)%	Stable
47	Silver	107	${}^{107}{Ag}$	106.905092(3)	51.839(8)%	Stable
		109	${}^{109}{Ag}$	108.904756(1)	48.161(8)%	Stable
48	Cadmium	114	${}^{114}{Cd}$	113.903365	28.754(81)%	Stable
49	Indium	115	${}^{115}{In}$	114.903879	95.719(52)%, $β^{-}$	$4.41(25) \times 10^{14} y$
50	Tin	120	${}^{120}{Sn}$	119.902203(1)	32.58(9)%	Stable
51	Antimony	121	${}^{121}{Sb}$	120.903811(3)	57.21(5)%	Stable
52	Tellurium	130	${}^{130}{Te}$	129.906223	34.08(62)%, double $β^{-}$	$7.91(21) \times 10^{20} y$
53	Iodine	127	${}^{127}I$	126.904473(4)	100%	Stable
		131	${}^{131}I$	130.906126(1)	$β^{-}$	8.0249(6) d
54	Xenon	132	${}^{132}{Xe}$	131.904155	26.909(55)%	Stable
		136	${}^{136}{Xe}$	135.907214	8.857(72)%, double $β^{-}$	$2.18(5) \times 10^{21} y$
55	Cesium	133	${}^{133}{Cs}$	132.905452	100%	Stable
		134	${}^{134}{Cs}$	133.906719	$β^{-}$	2.0650(4) y
56	Barium	137	${}^{137}{Ba}$	136.905827	11.23(23)%	Stable
		138	${}^{138}{Ba}$	137.905247	71.70(29)%	Stable
57	Lanthanum	139	${}^{139}{La}$	138.906363(1)	99.91119(71)%	Stable
58	Cerium	140	${}^{140}{Ce}$	139.905448(1)	88.450(51)%	Stable
59	Praseodymium	141	${}^{141}{Pr}$	140.907660(2)	100%	Stable
60	Neodymium	142	${}^{142}{Nd}$	141.907729(1)	27.152(40)%	Stable
61	Promethium	145	${}^{145}{Pm}$	144.912756(3)	EC	17.7(4) y
62	Samarium	152	${}^{152}{Sm}$	151.919739(1)	26.74(9)%	Stable
63	Europium	153	${}^{153}{Eu}$	152.921237(1)	52.19(6)%	Stable
64	Gadolinium	158	${}^{158}{Gd}$	157.924111(1)	24.820(20)%	Stable
65	Terbium	159	${}^{159}{Tb}$	158.925354(1)	100%	Stable
66	Dysprosium	164	${}^{164}{Dy}$	163.929181(1)	28.260(54)%	Stable
67	Holmium	165	${}^{165}{Ho}$	164.930329(1)	100%	Stable
68	Erbium	166	${}^{166}{Er}$	165.930301	33.503(36)%	Stable
69	Thulium	169	${}^{169}{Tm}$	168.934219(1)	100%	Stable
70	Ytterbium	174	${}^{174}{Yb}$	173.938868	31.896(26)%	Stable
71	Lutetium	175	${}^{175}{Lu}$	174.940777(1)	97.414(5)%	Stable
72	Hafnium	180	${}^{180}{Hf}$	179.946560(2)	35.12(16)%	Stable
73	Tantalum	181	${}^{181}{Ta}$	180.947999(2)	99.98824(23)%	Stable
74	Tungsten	184	${}^{184}W$	183.950933(1)	30.64(2)%	Stable
75	Rhenium	187	${}^{187}{Re}$	186.955752(1)	62.60(5)%, $β^{-}$	$4.160(2) \times 10^{10} y$
76	Osmium	191	${}^{191}{Os}$	190.960928(1)	$β^{-}$	14.99(2) d
		192	${}^{192}{Os}$	191.961479(2)	40.78(32)%	Stable
77	Iridium	191	${}^{191}{Ir}$	190.960591(1)	37.23(9)%	Stable
		193	${}^{193}{Ir}$	192.962924(1)	62.77(9)%	Stable
78	Platinum	195	${}^{195}{Pt}$	194.964794(1)	33.775(240)%	Stable
79	Gold	197	${}^{197}{Au}$	196.966570(1)	100%	Stable
		198	${}^{198}{Au}$	197.968244(1)	$β^{-}$	2.69464(14) d
80	Mercury	199	${}^{199}{Hg}$	198.968281(1)	16.94(12)%	Stable
		202	${}^{202}{Hg}$	201.970644(1)	29.74(13)%	Stable
81	Thallium	205	${}^{205}{Tl}$	204.974427(1)	70.41–70.56%	Stable
82	Lead	206	${}^{206}{Pb}$	205.974465(1)	1.90–86.73%	Stable
		207	${}^{207}{Pb}$	206.975897(1)	0.35–23.51%	Stable
		208	${}^{208}{Pb}$	207.976652(1)	3.38–97.75%	Stable
		210	${}^{210}{Pb}$	209.984188(2)	$β^{-}$	22.20(22) y
		211	${}^{211}{Pb}$	210.988735(2)	$β^{-}$	36.1628(25) min
		212	${}^{212}{Pb}$	211.991896(2)	$β^{-}$	10.627(6) h
83	Bismuth	209	${}^{209}{Bi}$	208.980399(1)	100%, $α$	$2.01(8) \times 10^{19} y$
		211	${}^{211}{Bi}$	210.987269(6)	$α$ , $β^{-}$	2.14(2) min
84	Polonium	210	${}^{210}{Po}$	209.982874(1)	$α$	138.376(2) d
85	Astatine	218	${}^{218}{At}$	218.008696(12)	$α$	1.28(6) s
86	Radon	222	${}^{222}{Rn}$	222.017576(2)	$α$	3.8215(2) d
87	Francium	223	${}^{223}{Fr}$	223.019734(2)	$β^{-}$	22.00(7) min
88	Radium	226	${}^{226}{Ra}$	226.025408(2)	$α$	$1.600(7) \times 10^{3} y$
89	Actinium	227	${}^{227}{Ac}$	227.027751(2)	$β^{-}$ , $α$	21.772(3) y
90	Thorium	228	${}^{228}{Th}$	228.028740(2)	$α$	1.9125(7) y
		232	${}^{232}{Th}$	232.038054(2)	99.98(2)%, $α$	$1.40(1) \times 10^{10} y$
91	Protactinium	231	${}^{231}{Pa}$	231.035883(2)	$α$	$3.265(20) \times 10^{4} y$
92	Uranium	233	${}^{233}U$	233.039634(2)	$α$	$1.5919(15) \times 10^{5} y$
		235	${}^{235}U$	235.043928(1)	0.7204(6)%, $α$	$7.04(1) \times 10^{8} y$
		236	${}^{236}U$	236.045566(1)	$α$	$2.342(4) \times 10^{7} y$
		238	${}^{238}U$	238.050787(2)	99.2742(10)%, $α$	$4.463(3) \times 10^{9} y$
		239	${}^{239}U$	239.054292(2)	$β^{-}$	23.45(2) min
93	Neptunium	239	${}^{239}{Np}$	239.052938(1)	$β^{-}$	2.356(3) d
94	Plutonium	239	${}^{239}{Pu}$	239.052162(1)	$α$	$2.411(3) \times 10^{4} y$
95	Americium	243	${}^{243}{Am}$	243.061380(1)	$α$	$7.350(9) \times 10^{3} y$
96	Curium	245	${}^{245}{Cm}$	245.065491(1)	$α$	$8.25(7) \times 10^{3} y$
97	Berkelium	247	${}^{247}{Bk}$	247.070306(6)	$α$	$1.38(25) \times 10^{3} y$
98	Californium	249	${}^{249}{Cf}$	249.074850(1)	$α$	351(2) y
99	Einsteinium	254	${}^{254}{Es}$	254.088024(3)	$α$	275.7(5) d
100	Fermium	253	${}^{253}{Fm}$	253.085181(2)	EC, $α$	3.00(12) d
101	Mendelevium	255	${}^{255}{Md}$	255.091082(6)	EC/ $β^{+}$ , $α$	27(2) min
102	Nobelium	255	${}^{255}{No}$	255.093196(15)	EC/ $β^{+}$ , $α$	3.52(18) min
103	Lawrencium	257	${}^{257}{Lr}$	257.099480(47)^#	$α$	6.0(4) s
104	Rutherfordium	261	$^{261} Rf$	261.108770(70)	SF, $α$	2.1(2) s
	Rutherfordium (metastable)	261	${}^{261 m}{Rf}$	261.10884(13)^#	$α$	74(5) s
105	Dubnium	262	$^{262} Db$	262.11407(15)^#	SF, $α$	34(4) s
106	Seaborgium	263	$^{263} Sg$	263.11830(10)^#	$α$ , SF	0.94(14) s
107	Bohrium	262	$^{262} Bh$	262.12265(10)	$α$	84(11) ms
108	Hassium	264	$^{264} Hs$	264.128356(31)	$α$ , SF	0.7(3) s
109	Meitnerium	266	$^{266} Mt$	266.13706(10)	$α$	2.0(5) ms

Table notes

Masses are for neutral atoms in their nuclear ground states, except for the free neutron and the explicitly labeled metastable state. Sources: AME2020 (atomic masses), NUBASE2020 (half-lives and decay modes), and CIAAW 2024 (natural isotopic compositions). Only selected isotopes are listed, so an element’s listed abundances need not sum to 100%.

Masses are rounded to at most six decimal places. Parentheses give the uncertainty in the last quoted digits; for example, 3.52(18) min means 3.52 ± 0.18 min. Mass uncertainties smaller than half the last displayed unit are omitted. A superscript # marks a mass estimated from systematic trends. Carbon-12 has mass exactly 12 u by definition. Ranges describe natural variation in normal terrestrial materials; other abundances are representative values with their quoted uncertainties. Unusual materials can fall outside these ranges.

Only confirmed decay branches with evaluated fractions of at least 0.1% are listed. EC means electron capture, SF means spontaneous fission, and double

β^{-}

means double-beta decay. EC/

β^{+}

denotes a combined electron-capture/positron-emission branch when NUBASE does not separate the two contributions; it does not establish a separate branching fraction for each. “Stable” follows the evaluation and does not imply that every energetically possible decay has been excluded.

The superscript m identifies a metastable nuclear state. For rutherfordium-261, the 2.1(2) s and 74(5) s entries follow the ground-state and isomer assignments in NUBASE2020, which flags their ordering as uncertain. Ground-state/isomer ordering is also uncertain for lawrencium-257 and seaborgium-263; the rows here follow the evaluation’s ground-state assignments. The rutherfordium-261m mass is estimated from the AME2020 ground-state mass and the NUBASE2020 excitation energy, with uncertainty based on the evaluated isomer mass excess.