Standard Atomic Weights of the Elements 2009 Based on relative atomic mass of 12C  12, where 12C is a neutral atom in its nuclear and electronic ground state.†
Name
Actinium*
Aluminum
Americium*
Antimony
Argon
Arsenic
Astatine*
Barium
Berkelium*
Beryllium
Bismuth
Bohrium*
Boron
Bromine
Cadmium
Cesium
Calcium
Californium*
Carbon
Cerium
Chlorine
Chromium
Cobalt
Copernicium*
Copper
Curium*
Darmstadtium*
Dubnium*
Dysprosium
Einsteinium*
Erbium
Europium
Fermium*
Flerovium*
Fluorine
Francium*
Gadolinium
Gallium
Germanium
Symbol
Atomic
Number
Ac
Al
Am
Sb
Ar
As
At
Ba
Bk
Be
Bi
Bh
B
Br
Cd
Cs
Ca
Cf
C
Ce
Cl
Cr
Co
Cn
Cu
Cm
Ds
Db
Dy
Es
Er
Eu
Fm
Fl
F
Fr
Gd
Ga
Ge
89
13
95
51
18
33
85
56
97
4
83
107
5
35
48
55
20
98
6
58
17
24
27
112
29
96
110
105
66
99
68
63
100
114
9
87
64
31
32
Atomic
Weight
(227)
26.9815386(8)
(243)
121.760(1)
39.948(1)
74.92160(2)
(210)
137.327(7)
(247)
9.012182(3)
208.98040(1)
(272)
10.806; 10.821
79.904(1)
112.411(8)
132.9054519(2)
40.078(4)
(251)
12.0096; 12.0116
140.116(1)
35.446; 35.457
51.9961(6)
58.933195(5)
(285)
63.546(3)
(247)
(281)
(268)
162.500(1)
(252)
167.259(3)
151.964(1)
(257)
(289)
18.9984032(5)
(223)
157.25(3)
69.723(1)
72.64(1)
Name
Gold
Hafnium
Hassium*
Helium
Holmium
Hydrogen
Indium
Iodine
Iridium
Iron
Krypton
Lanthanum
Lawrencium*
Lead
Lithium
Livermorium*
Lutetium
Magnesium
Manganese
Meitnerium*
Mendelevium*
Mercury
Molybdenum
Neodymium
Neon
Neptunium*
Nickel
Niobium
Nitrogen
Nobelium*
Osmium
Oxygen
Palladium
Phosphorus
Platinum
Plutonium*
Polonium*
Potassium
Praseodymium
Symbol
Atomic
Number
Atomic
Weight
Au
Hf
Hs
He
Ho
H
In
I
Ir
Fe
Kr
La
Lr
Pb
Li
Lv
Lu
Mg
Mn
Mt
Md
Hg
Mo
Nd
Ne
Np
Ni
Nb
N
No
Os
O
Pd
P
Pt
Pu
Po
K
Pr
79
72
108
2
67
1
49
53
77
26
36
57
103
82
3
116
71
12
25
109
101
80
42
60
10
93
28
41
7
102
76
8
46
15
78
94
84
19
59
196.966569(4)
178.49(2)
(277)
4.002602(2)
164.93032(2)
1.00784; 1.00811
114.818(3)
126.90447(3)
192.217(3)
55.845(2)
83.798(2)
138.90547(7)
(262)
207.2(1)
6.938; 6.997
(293)
174.9668(1)
24.3050(6)
54.938045(5)
(276)
(258)
200.59(2)
95.96(2)
144.242(3)
20.1797(6)
(237)
58.6934(4)
92.90638(2)
14.00643; 14.00728
(259)
190.23(3)
15.99903; 15.99977
106.42(1)
30.973762(2)
195.084(9)
(244)
(209)
39.0983(1)
140.90765(2)
Name
Promethium*
Protactinium*
Radium*
Radon*
Rhenium
Rhodium
Roentgenium*
Rubidium
Ruthenium
Rutherfordium*
Samarium
Scandium
Seaborgium*
Selenium
Silicon
Silver
Sodium
Strontium
Sulfur
Tantalum
Technetium*
Tellurium
Terbium
Thallium
Thorium*
Thulium
Tin
Titanium
Tungsten
Ununoctium*
Ununpentium*
Ununseptium*
Ununtrium*
Uranium*
Vanadium
Xenon
Ytterbium
Yttrium
Zinc
Zirconium
Symbol
Atomic
Number
Pm
Pa
Ra
Rn
Re
Rh
Rg
Rb
Ru
Rf
Sm
Sc
Sg
Se
Si
Ag
Na
Sr
S
Ta
Tc
Te
Tb
Tl
Th
Tm
Sn
Ti
W
Uuo
Uup
Uus
Uut
U
V
Xe
Yb
Y
Zn
Zr
61
91
88
86
75
45
111
37
44
104
62
21
106
34
14
47
11
38
16
73
43
52
65
81
90
69
50
22
74
118
115
117
113
92
23
54
70
39
30
40
Atomic
Weight
(145)
231.03588(2)
(226)
(222)
186.207(1)
102.90550(2)
(280)
85.4678(3)
101.07(2)
(265)
150.36(2)
44.955912(6)
(271)
78.96(3)
28.084; 28.086
107.8682(2)
22.98976928(2)
87.62(1)
32.059; 32.076
180.94788(2)
(98)
127.60(3)
158.92535(2)
204.382; 204.385
232.03806(2)
168.93421(2)
118.710(7)
47.867(1)
183.84(1)
(294)
(288)
(294)
284.178
238.02891(3)
50.9415(1)
131.293(6)
173.054(5)
88.90585(2)
65.38(2)
91.224(2)
†Standard atomic weights for elements with two or more isotopes are not constants of nature. Atomic weights for elements with two or more stable isotopes are given with uncertainties following the last significant figure to which they are attributed. For ten of these
­elements, variations in atomic weight exceed the uncertainty, so an atomic-weight interval is given as [a; b], where a and b denote the lower and upper bounds, respectively, of the atomic weight interval for the element. The interval encompasses atomic-weight values of
all normal materials, regardless of origin or treatment of the sample. (Atomic weights on the periodic table for these ten elements are single values that take into consideration the variation in atomic weights.)
*Elements with no stable nuclide; the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such ele­ments (Th, Pa, and U) have a characteristic terrestial isotopic composition, and the atomic weight is tabulated for
these. http://www.chem.qmul.ac.uk/iupac/AtWt/