ATOMIC WEIGHTS OF THE ELEMENTS 1999

World Wide Web version of atomic weight data prepared by G. P. Moss, originally from a file provided by D. R. Lide.

Note The claim [Phys. Rev. Lett. 1999, 83, 1104] for elements 116 and 118 has been withdrawn [see Chem. Eng. News 2001, 79, 10 (6 August 2001); Phys. Rev. Lett. in press]

Previous values may be consulted from the 1993 table, the 1995 table or 1997 table

The original paper should be consulted for details of the half life of the radioisotopes quoted below. In addition there are recommendations on the scales for quoting relative isotope-ratio data for stable hydrogen, lithium, carbon, oxygen and sulfur. Also there is a report on the different isotopic compositions of some non-terrestrial materials.

A number in parentheses indicates the uncertainty in the last digit of the atomic weight.

See also a copy of the periodic table with atomic weights to five significant figures.

See also below the list in Name order.

At No	Symbol	Name	Atomic Wt	Notes
1	H	Hydrogen	1.00794(7)	1, 2, 3
2	He	Helium	4.002602(2)	1, 2
3	Li	Lithium	[6.941(2)]	1, 2, 3, 4
4	Be	Beryllium	9.012182(3)
5	B	Boron	10.811(7)	1, 2, 3
6	C	Carbon	12.0107(8)	1, 2
7	N	Nitrogen	14.0067(2)	1, 2
8	O	Oxygen	15.9994(3)	1, 2
9	F	Fluorine	18.9984032(5)
10	Ne	Neon	20.1797(6)	1, 3
11	Na	Sodium	22.989770(2)
12	Mg	Magnesium	24.3050(6)
13	Al	Aluminium	26.981538(2)
14	Si	Silicon	28.0855(3)	2
15	P	Phosphorus	30.973761(2)
16	S	Sulfur	32.065(5)	1, 2
17	Cl	Chlorine	35.453(2)	3
18	Ar	Argon	39.948(1)	1, 2
19	K	Potassium	39.0983(1)	1
20	Ca	Calcium	40.078(4)	1
21	Sc	Scandium	44.955910(8)
22	Ti	Titanium	47.867(1)
23	V	Vanadium	50.9415(1)
24	Cr	Chromium	51.9961(6)
25	Mn	Manganese	54.938049(9)
26	Fe	Iron	55.845(2)
27	Co	Cobalt	58.933200(9)
28	Ni	Nickel	58.6934(2)
29	Cu	Copper	63.546(3)	2
30	Zn	Zinc	65.39(2)	7
31	Ga	Gallium	69.723(1)
32	Ge	Germanium	72.64(1)
33	As	Arsenic	74.92160(2)
34	Se	Selenium	78.96(3)
35	Br	Bromine	79.904(1)
36	Kr	Krypton	83.80(1)	1, 3, 7
37	Rb	Rubidium	85.4678(3)	1
38	Sr	Strontium	87.62(1)	1, 2
39	Y	Yttrium	88.90585(2)
40	Zr	Zirconium	91.224(2)	1
41	Nb	Niobium	92.90638(2)
42	Mo	Molybdenum	95.94(1)	1, 7
43	Tc	Technetium	[98]	5
44	Ru	Ruthenium	101.07(2)	1
45	Rh	Rhodium	102.90550(2)
46	Pd	Palladium	106.42(1)	1
47	Ag	Silver	107.8682(2)	1
48	Cd	Cadmium	112.411(8)	1
49	In	Indium	114.818(3)
50	Sn	Tin	118.710(7)	1
51	Sb	Antimony	121.760(1)	1
52	Te	Tellurium	127.60(3)	1
53	I	Iodine	126.90447(3)
54	Xe	Xenon	131.293(6)	1, 3
55	Cs	Caesium	132.90545(2)
56	Ba	Barium	137.327(7)
57	La	Lanthanum	138.9055(2)	1
58	Ce	Cerium	140.116(1)	1
59	Pr	Praseodymium	140.90765(2)
60	Nd	Neodymium	144.24(3)	1
61	Pm	Promethium	[145]	5
62	Sm	Samarium	150.36(3)	1
63	Eu	Europium	151.964(1)	1
64	Gd	Gadolinium	157.25(3)	1
65	Tb	Terbium	158.92534(2)
66	Dy	Dysprosium	162.50(3)	1, 7
67	Ho	Holmium	164.93032(2)
68	Er	Erbium	167.259(3)	1
69	Tm	Thulium	168.93421(2)
70	Yb	Ytterbium	173.04(3)	1
71	Lu	Lutetium	174.967(1)	1
72	Hf	Hafnium	178.49(2)
73	Ta	Tantalum	180.9479(1)
74	W	Tungsten	183.84(1)
75	Re	Rhenium	186.207(1)
76	Os	Osmium	190.23(3)	1
77	Ir	Iridium	192.217(3)
78	Pt	Platinum	195.078(2)
79	Au	Gold	196.96655(2)
80	Hg	Mercury	200.59(2)
81	Tl	Thallium	204.3833(2)
82	Pb	Lead	207.2(1)	1, 2
83	Bi	Bismuth	208.98038(2)
84	Po	Polonium	[209]	5
85	At	Astatine	[210]	5
86	Rn	Radon	[222]	5
87	Fr	Francium	[223]	5
88	Ra	Radium	[226]	5
89	Ac	Actinium	[227]	5
90	Th	Thorium	232.0381(1)	1, 5
91	Pa	Protactinium	231.03588(2)	5
92	U	Uranium	238.02891(3)	1, 3, 5
93	Np	Neptunium	[237]	5
94	Pu	Plutonium	[244]	5
95	Am	Americium	[243]	5
96	Cm	Curium	[247]	5
97	Bk	Berkelium	[247]	5
98	Cf	Californium	[251]	5
99	Es	Einsteinium	[252]	5
100	Fm	Fermium	[257]	5
101	Md	Mendelevium	[258]	5
102	No	Nobelium	[259]	5
103	Lr	Lawrencium	[262]	5
104	Rf	Rutherfordium	[261]	5, 6
105	Db	Dubnium	[262]	5, 6
106	Sg	Seaborgium	[266]	5, 6
107	Bh	Bohrium	[264]	5, 6
108	Hs	Hassium	[277]	5, 6
109	Mt	Meitnerium	[268]	5, 6
110	Uun	Ununnilium	[281]	5, 6
111	Uuu	Unununium	[272]	5, 6
112	Uub	Ununbium	[285]	5, 6
114	Uuq	Ununquadium	[289]	5, 6
116	Uuh	Ununhexium		see Note above
118	Uuo	Ununoctium		see Note above

1. Geological specimens 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 specimens and that given in the Table may exceed the stated uncertainty.
2. Range in isotopic composition of normal terrestrial material prevents a more precise value being given; the tabulated value should be applicable to any normal material.
3. Modified isotopic compositions may be found in commercially available material because it has been subject to an undisclosed or inadvertant isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the Table can occur.
4. Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accurate value is required, it must be determined for the specific material [range quoted for 1995 table 6.94 and 6.99].
5. Element has no stable nuclides. The value enclosed in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated.
6. The names and symbols for elements 110-118 are under review. The temporary system recommended by J Chatt, Pure Appl. Chem., 51, 381-384 (1979) is used above. The names of elements 101-109 were agreed in 1997 (See Pure Appl. Chem., 1997, 69, 2471-2473).
7. The 2001 review agreed the following changes zinc 65.39(2) to 65.409(4), krypton 83.80(1) to 83.798(2), molybdenum 95.94(1) to 95.94(2) and dysprosium 162.50(3) to 162.500(1).

At No	Symbol	Name	Atomic Wt	Notes
89	Ac	Actinium	[227]	5
13	Al	Aluminium	26.981538(2)
95	Am	Americium	[243]	5
51	Sb	Antimony	121.760(1)	1
18	Ar	Argon	39.948(1)	1, 2
33	As	Arsenic	74.92160(2)
85	At	Astatine	[210]	5
56	Ba	Barium	137.327(7)
97	Bk	Berkelium	[247]	5
4	Be	Beryllium	9.012182(3)
83	Bi	Bismuth	208.98038(2)
107	Bh	Bohrium	[264]	5, 6
5	B	Boron	10.811(7)	1, 2, 3
35	Br	Bromine	79.904(1)
48	Cd	Cadmium	112.411(8)	1
55	Cs	Caesium	132.90545(2)
20	Ca	Calcium	40.078(4)	1
98	Cf	Californium	[251]	5
6	C	Carbon	12.0107(8)	1, 2
58	Ce	Cerium	140.116(1)	1
17	Cl	Chlorine	35.453(2)	3
24	Cr	Chromium	51.9961(6)
27	Co	Cobalt	58.933200(9)
29	Cu	Copper	63.546(3)	2
96	Cm	Curium	[247]	5
105	Db	Dubnium	[262]	5, 6
66	Dy	Dysprosium	162.50(3)	1, 7
99	Es	Einsteinium	[252]	5
68	Er	Erbium	167.259(3)	1
63	Eu	Europium	151.964(1)	1
100	Fm	Fermium	[257]	5
9	F	Fluorine	18.9984032(5)
87	Fr	Francium	[223]	5
64	Gd	Gadolinium	157.25(3)	1
31	Ga	Gallium	69.723(1)
32	Ge	Germanium	72.64(1)
79	Au	Gold	196.96655(2)
72	Hf	Hafnium	178.49(2)
108	Hs	Hassium	[277]	5, 6
2	He	Helium	4.002602(2)	1, 2
67	Ho	Holmium	164.93032(2)
1	H	Hydrogen	1.00794(7)	1, 2, 3
49	In	Indium	114.818(3)
53	I	Iodine	126.90447(3)
77	Ir	Iridium	192.217(3)
26	Fe	Iron	55.845(2)
36	Kr	Krypton	83.80(1)	1, 3, 7
57	La	Lanthanum	138.9055(2)	1
103	Lr	Lawrencium	[262]	5
82	Pb	Lead	207.2(1)	1, 2
3	Li	Lithium	[6.941(2)]	1, 2, 3, 4
71	Lu	Lutetium	174.967(1)	1
12	Mg	Magnesium	24.3050(6)
25	Mn	Manganese	54.938049(9)
109	Mt	Meitnerium	[268]	5, 6
101	Md	Mendelevium	[258]	5
80	Hg	Mercury	200.59(2)
42	Mo	Molybdenum	95.94(1)	1, 7
60	Nd	Neodymium	144.24(3)	1
10	Ne	Neon	20.1797(6)	1, 3
93	Np	Neptunium	[237]	5
28	Ni	Nickel	58.6934(2)
41	Nb	Niobium	92.90638(2)
7	N	Nitrogen	14.0067(2)	1, 2
102	No	Nobelium	[259]	5
76	Os	Osmium	190.23(3)	1
8	O	Oxygen	15.9994(3)	1, 2
46	Pd	Palladium	106.42(1)	1
15	P	Phosphorus	30.973761(2)
78	Pt	Platinum	195.078(2)
94	Pu	Plutonium	[244]	5
84	Po	Polonium	[209]	5
19	K	Potassium	39.0983(1)	1
59	Pr	Praseodymium	140.90765(2)
61	Pm	Promethium	[145]	5
91	Pa	Protactinium	231.03588(2)	5
88	Ra	Radium	[226]	5
86	Rn	Radon	[222]	5
75	Re	Rhenium	186.207(1)
45	Rh	Rhodium	102.90550(2)
37	Rb	Rubidium	85.4678(3)	1
44	Ru	Ruthenium	101.07(2)	1
104	Rf	Rutherfordium	[261]	5, 6
62	Sm	Samarium	150.36(3)	1
21	Sc	Scandium	44.955910(8)
106	Sg	Seaborgium	[266]	5, 6
34	Se	Selenium	78.96(3)
14	Si	Silicon	28.0855(3)	2
47	Ag	Silver	107.8682(2)	1
11	Na	Sodium	22.989770(2)
38	Sr	Strontium	87.62(1)	1, 2
16	S	Sulfur	32.065(5)	1, 2
73	Ta	Tantalum	180.9479(1)
43	Tc	Technetium	[98]	5
52	Te	Tellurium	127.60(3)	1
65	Tb	Terbium	158.92534(2)
81	Tl	Thallium	204.3833(2)
90	Th	Thorium	232.0381(1)	1, 5
69	Tm	Thulium	168.93421(2)
50	Sn	Tin	118.710(7)	1
22	Ti	Titanium	47.867(1)
74	W	Tungsten	183.84(1)
112	Uub	Ununbium	[285]	5, 6
116	Uuh	Ununhexium		see Note above
110	Uun	Ununnilium	[281]	5, 6
118	Uuo	Ununoctium		see Note above
114	Uuq	Ununquadium	[289]	5, 6
111	Uuu	Unununium	[272]	5, 6
92	U	Uranium	238.02891(3)	1, 3, 5
23	V	Vanadium	50.9415(1)
54	Xe	Xenon	131.293(6)	1, 3
70	Yb	Ytterbium	173.04(3)	1
39	Y	Yttrium	88.90585(2)
30	Zn	Zinc	65.39(2)	7
40	Zr	Zirconium	91.224(2)	1

1. Geological specimens 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 specimens and that given in the Table may exceed the stated uncertainty.
2. Range in isotopic composition of normal terrestrial material prevents a more precise value being given; the tabulated value should be applicable to any normal material.
3. Modified isotopic compositions may be found in commercially available material because it has been subject to an undisclosed or inadvertant isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the Table can occur.
4. Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accurate value is required, it must be determined for the specific material [range quoted for 1995 table 6.94 and 6.99].
5. Element has no stable nuclides. The value enclosed in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated.
6. The names and symbols for elements 110-112 are under review. The temporary system recommended by J Chatt, Pure Appl. Chem., 51, 381-384 (1979) is used above. The names of elements 101-109 were agreed in 1997 (See Pure Appl. Chem., 1997, 69, 2471-2473)
7. The 2001 review agreed the following changes zinc 65.39(2) to 65.409(4), krypton 83.80(1) to 83.798(2), molybdenum 95.94(1) to 95.94(2) and dysprosium 162.50(3) to 162.500(1).