isotope
, one of two or more species of
atoms
of a
chemical element
with the same
atomic number
and position in the
periodic table
and nearly identical chemical behaviour but with different
atomic masses
and physical properties. Every chemical element has one or more isotopes.
An
atom
is first identified and labeled according to the number of
protons
in its nucleus. This
atomic number
is ordinarily given the symbol
Z
. The great importance of the atomic number derives from the observation that all atoms with the same atomic number have nearly, if not precisely, identical chemical properties. A large collection of atoms with the same atomic number
constitutes
a sample of an element. A bar of pure
uranium
, for instance, would consist entirely of atoms with atomic number 92. The periodic table of the elements assigns one place to every atomic number, and each of these places is labeled with the common name of the element, as, for example,
calcium
,
radon
, or uranium.
Not all the atoms of an element need have the same number of
neutrons
in their nuclei. In fact, it is precisely the variation in the number of neutrons in the nuclei of atoms that gives rise to isotopes.
Hydrogen
is a case in point. It has the atomic number 1. Three nuclei with one
proton
are known that contain 0, 1, and 2 neutrons, respectively. The three share the place in the periodic table assigned to atomic number 1 and hence are called isotopes (from the Greek
isos
, meaning “same,” and
topos
, signifying “place”) of hydrogen.
Many important properties of an isotope depend on its mass. The total number of neutrons and protons (symbol
A
), or
mass number
, of the nucleus gives approximately the mass measured on the so-called atomic-mass-unit (amu) scale. The numerical difference between the actual measured mass of an isotope and
A
is called either the mass excess or the
mass defect
(symbol Δ;
see
table).
Britannica Quiz
Facts You Should Know: The Periodic Table Quiz
Abundances of the isotopes
element
|
Z
|
symbol
|
A
|
abundance
|
mass excess
|
Sources: G. Audi and A.H. Wapstra, "The 1995 Update to Atomic Mass Evaluation," Nuclear Physics, A595:409?480 (1995); K.J.R. Rosman and P.D.P. Taylor, "Isotopic Compositions of the Elements 1997," J. Phys. Chem. Ref. Data, 27:1275?85 (1995).
|
hydrogen
|
1
|
H
|
1
|
99.9885
|
7.289
|
|
|
|
2
|
0.0151
|
13.136
|
helium
|
2
|
He
|
3
|
0.000138
|
14.931
|
|
|
|
4
|
99.999863
|
2.425
|
lithium
|
3
|
Li
|
6
|
7.59
|
14.086
|
|
|
|
7
|
92.41
|
14.908
|
beryllium
|
4
|
Be
|
9
|
100
|
11.348
|
boron
|
5
|
B
|
10
|
19.9
|
12.051
|
|
|
|
11
|
80.1
|
8.668
|
carbon
|
6
|
C
|
12
|
98.93
|
0
|
|
|
|
13
|
1.07
|
3.125
|
nitrogen
|
7
|
N
|
14
|
99.632
|
2.863
|
|
|
|
15
|
0.368
|
0.101
|
oxygen
|
8
|
O
|
16
|
99.757
|
?4.737
|
|
|
|
17
|
0.038
|
?0.809
|
|
|
|
18
|
0.205
|
?0.782
|
fluorine
|
9
|
F
|
19
|
100
|
?1.487
|
neon
|
10
|
Ne
|
20
|
90.48
|
?7.042
|
|
|
|
21
|
0.27
|
?5.732
|
|
|
|
22
|
9.25
|
?8.024
|
sodium
|
11
|
Na
|
23
|
100
|
?9.529
|
magnesium
|
12
|
Mg
|
24
|
78.99
|
?13.933
|
|
|
|
25
|
10.00
|
?13.193
|
|
|
|
26
|
11.01
|
?16.214
|
aluminum
|
13
|
Al
|
27
|
100
|
?17.197
|
silicon
|
14
|
Si
|
28
|
92.2297
|
?21.493
|
|
|
|
29
|
4.6832
|
?21.895
|
|
|
|
30
|
3.0872
|
?24.433
|
phosphorus
|
15
|
P
|
31
|
100
|
?24.441
|
sulfur
|
16
|
S
|
32
|
94.93
|
?26.016
|
|
|
|
33
|
0.76
|
?26.586
|
|
|
|
34
|
4.29
|
?29.932
|
|
|
|
36
|
0.02
|
?30.664
|
chlorine
|
17
|
Cl
|
35
|
75.78
|
?29.014
|
|
|
|
37
|
24.22
|
?31.762
|
argon
|
18
|
Ar
|
36
|
0.3365
|
?30.230
|
|
|
|
38
|
0.0632
|
?34.715
|
|
|
|
40
|
99.6003
|
?35.040
|
potassium
|
19
|
K
|
39
|
93.2581
|
?33.807
|
|
|
|
40
|
0.0117
|
?33.535
|
|
|
|
41
|
6.7302
|
?35.559
|
calcium
|
20
|
Ca
|
40
|
96.941
|
?34.846
|
|
|
|
42
|
0.647
|
?38.547
|
|
|
|
43
|
0.135
|
?38.408
|
|
|
|
44
|
2.086
|
?41.469
|
|
|
|
46
|
0.004
|
?43.135
|
|
|
|
48
|
0.187
|
?44.215
|
scandium
|
21
|
Sc
|
45
|
100
|
?41.069
|
titanium
|
22
|
Ti
|
46
|
8.25
|
?44.125
|
|
|
|
47
|
7.44
|
?44.932
|
|
|
|
48
|
73.72
|
?48.487
|
|
|
|
49
|
5.41
|
?48.558
|
|
|
|
50
|
5.18
|
?51.426
|
vanadium
|
23
|
V
|
50
|
0.250
|
?49.218
|
|
|
|
51
|
99.750
|
?52.198
|
chromium
|
24
|
Cr
|
50
|
4.345
|
?50.254
|
|
|
|
52
|
83.789
|
?55.413
|
|
|
|
53
|
9.501
|
?55.281
|
|
|
|
54
|
2.365
|
?56.928
|
manganese
|
25
|
Mn
|
55
|
100
|
?57.706
|
iron
|
26
|
Fe
|
54
|
5.845
|
?56.248
|
|
|
|
56
|
91.754
|
?60.601
|
|
|
|
57
|
2.119
|
?60.176
|
|
|
|
58
|
0.282
|
?62.149
|
cobalt
|
27
|
Co
|
59
|
100
|
?62.224
|
nickel
|
28
|
Ni
|
58
|
68.0769
|
?60.223
|
|
|
|
60
|
26.2231
|
?64.468
|
|
|
|
61
|
1.1399
|
?64.217
|
|
|
|
62
|
3.6345
|
?66.743
|
|
|
|
64
|
0.9256
|
?67.096
|
copper
|
29
|
Cu
|
63
|
69.17
|
?65.576
|
|
|
|
65
|
30.83
|
?67.260
|
zinc
|
30
|
Zn
|
64
|
48.63
|
?66.000
|
|
|
|
66
|
27.90
|
?68.896
|
|
|
|
67
|
4.10
|
?67.877
|
|
|
|
68
|
18.75
|
?70.004
|
|
|
|
70
|
0.62
|
?69.559
|
gallium
|
31
|
Ga
|
69
|
60.108
|
?69.321
|
|
|
|
71
|
39.892
|
?70.137
|
germanium
|
32
|
Ge
|
70
|
20.84
|
?70.560
|
|
|
|
72
|
27.54
|
?72.586
|
|
|
|
73
|
7.73
|
?71.299
|
|
|
|
74
|
36.28
|
?73.422
|
|
|
|
76
|
7.61
|
?73.213
|
arsenic
|
33
|
As
|
75
|
100
|
?73.032
|
selenium
|
34
|
Se
|
74
|
0.89
|
?72.213
|
|
|
|
76
|
9.37
|
?75.252
|
|
|
|
77
|
7.63
|
?74.599
|
|
|
|
78
|
23.77
|
?77.026
|
|
|
|
80
|
49.61
|
?77.759
|
|
|
|
82
|
8.73
|
?77.593
|
bromine
|
35
|
Br
|
79
|
50.69
|
?76.068
|
|
|
|
81
|
49.31
|
?77.974
|
krypton
|
36
|
Kr
|
78
|
0.35
|
?74.160
|
|
|
|
80
|
2.28
|
?77.893
|
|
|
|
82
|
11.58
|
?80.589
|
|
|
|
83
|
11.49
|
?79.982
|
|
|
|
84
|
57.00
|
?82.431
|
|
|
|
86
|
17.30
|
?83.266
|
rubidium
|
37
|
Rb
|
85
|
72.17
|
?82.168
|
|
|
|
87
|
27.83
|
?84.595
|
strontium
|
38
|
Sr
|
84
|
0.56
|
?80.644
|
|
|
|
86
|
9.86
|
?84.522
|
|
|
|
87
|
7.00
|
?84.878
|
|
|
|
88
|
82.58
|
?87.920
|
yttrium
|
39
|
Y
|
89
|
100
|
?87.702
|
zirconium
|
40
|
Zr
|
90
|
51.45
|
?88.768
|
|
|
|
91
|
11.22
|
?87.891
|
|
|
|
92
|
17.15
|
?88.455
|
|
|
|
94
|
17.38
|
?87.266
|
|
|
|
96
|
2.80
|
?85.441
|
niobium
|
41
|
Nb
|
93
|
100
|
?87.209
|
molybdenum
|
42
|
Mo
|
92
|
14.84
|
?86.805
|
|
|
|
94
|
9.25
|
?88.410
|
|
|
|
95
|
15.92
|
?87.708
|
|
|
|
96
|
16.68
|
?88.791
|
|
|
|
97
|
9.55
|
?87.541
|
|
|
|
98
|
24.13
|
?88.112
|
|
|
|
100
|
9.63
|
?86.184
|
ruthenium
|
44
|
Ru
|
96
|
5.54
|
?86.072
|
|
|
|
98
|
1.87
|
?88.224
|
|
|
|
99
|
12.76
|
?87.617
|
|
|
|
100
|
12.60
|
?89.219
|
|
|
|
101
|
17.06
|
?87.950
|
|
|
|
102
|
31.55
|
?89.098
|
|
|
|
104
|
18.62
|
?88.091
|
rhodium
|
45
|
Rh
|
103
|
100
|
?88.022
|
palladium
|
46
|
Pd
|
102
|
1.020
|
?87.926
|
|
|
|
104
|
11.14
|
?89.391
|
|
|
|
105
|
22.33
|
?88.414
|
|
|
|
106
|
27.33
|
?89.905
|
|
|
|
108
|
26.46
|
?89.522
|
|
|
|
110
|
11.72
|
?88.350
|
silver
|
47
|
Ag
|
107
|
51.8392
|
?88.405
|
|
|
|
109
|
48.1608
|
?88.720
|
cadmium
|
48
|
Cd
|
106
|
1.25
|
?87.134
|
|
|
|
108
|
0.89
|
?89.253
|
|
|
|
110
|
12.49
|
?90.350
|
|
|
|
111
|
12.80
|
?89.254
|
|
|
|
112
|
24.13
|
?90.581
|
|
|
|
113
|
12.22
|
?89.050
|
|
|
|
114
|
28.73
|
?90.021
|
|
|
|
116
|
7.49
|
?88.720
|
indium
|
49
|
In
|
113
|
4.288
|
?89.366
|
|
|
|
115
|
95.712
|
?89.537
|
tin
|
50
|
Sn
|
112
|
0.973
|
?88.659
|
|
|
|
114
|
0.659
|
?90.558
|
|
|
|
115
|
0.339
|
?90.033
|
|
|
|
116
|
14.536
|
?91.525
|
|
|
|
117
|
7.676
|
?90.398
|
|
|
|
118
|
24.223
|
?91.653
|
|
|
|
119
|
8.585
|
?90.067
|
|
|
|
120
|
32.593
|
?91.103
|
|
|
|
122
|
4.629
|
?89.944
|
|
|
|
124
|
5.789
|
?88.236
|
antimony
|
51
|
Sb
|
121
|
57.213
|
?89.593
|
|
|
|
123
|
42.787
|
?89.222
|
tellurium
|
52
|
Te
|
120
|
0.096
|
?89.405
|
|
|
|
122
|
2.603
|
?90.311
|
|
|
|
123
|
0.908
|
?89.169
|
|
|
|
124
|
4.816
|
?90.523
|
|
|
|
125
|
7.139
|
?89.028
|
|
|
|
126
|
18.952
|
?90.070
|
|
|
|
128
|
31.687
|
?88.994
|
|
|
|
130
|
33.799
|
?87.353
|
iodine
|
53
|
I
|
127
|
100
|
?88.987
|
xenon
|
54
|
Xe
|
124
|
0.08913
|
?87.658
|
|
|
|
126
|
0.08880
|
?89.173
|
|
|
|
128
|
1.91732
|
?89.861
|
|
|
|
129
|
26.43964
|
?89.697
|
|
|
|
130
|
4.08271
|
?89.881
|
|
|
|
131
|
21.17961
|
?88.416
|
|
|
|
132
|
26.89157
|
?89.280
|
|
|
|
134
|
10.44232
|
?88.124
|
|
|
|
136
|
8.86590
|
?86.424
|
cesium
|
55
|
Cs
|
133
|
100
|
?88.076
|
barium
|
56
|
Ba
|
130
|
0.1058
|
?87.271
|
|
|
|
132
|
0.1012
|
?88.440
|
|
|
|
134
|
2.417
|
?88.954
|
|
|
|
135
|
6.592
|
?87.856
|
|
|
|
136
|
7.853
|
?88.892
|
|
|
|
137
|
11.232
|
?87.727
|
|
|
|
138
|
71.699
|
?88.267
|
lanthanum
|
57
|
La
|
138
|
0.09017
|
?86.529
|
|
|
|
139
|
99.91
|
?87.236
|
cerium
|
58
|
Ce
|
136
|
0.186
|
?86.500
|
|
|
|
138
|
0.251
|
?87.574
|
|
|
|
140
|
88.449
|
?88.088
|
|
|
|
142
|
11.114
|
?84.543
|
praseodymium
|
59
|
Pr
|
141
|
100
|
?86.026
|
neodymium
|
60
|
Nd
|
142
|
27.16
|
?85.960
|
|
|
|
143
|
12.18
|
?84.012
|
|
|
|
144
|
23.83
|
?83.758
|
|
|
|
145
|
8.30
|
?81.442
|
|
|
|
146
|
17.17
|
?80.936
|
|
|
|
148
|
5.74
|
?77.418
|
|
|
|
150
|
5.62
|
?73.694
|
samarium
|
62
|
Sm
|
144
|
3.0734
|
?81.976
|
|
|
|
147
|
14.9934
|
?79.276
|
|
|
|
148
|
11.2406
|
?79.347
|
|
|
|
149
|
13.8189
|
?77.147
|
|
|
|
150
|
7.3796
|
?77.061
|
|
|
|
152
|
26.7421
|
?74.773
|
|
|
|
154
|
22.7520
|
?72.465
|
europium
|
63
|
Eu
|
151
|
47.810
|
?74.663
|
|
|
|
153
|
52.190
|
?73.377
|
gadolinium
|
64
|
Gd
|
152
|
0.2029
|
?74.717
|
|
|
|
154
|
2.1809
|
?73.716
|
|
|
|
155
|
14.7998
|
?72.080
|
|
|
|
156
|
20.4664
|
?72.545
|
|
|
|
157
|
15.6518
|
?70.834
|
|
|
|
158
|
24.8347
|
?0.700
|
|
|
|
160
|
21.8635
|
?67.952
|
terbium
|
65
|
Tb
|
159
|
100
|
?69.542
|
dysprosium
|
66
|
Dy
|
156
|
0.056
|
?70.534
|
|
|
|
158
|
0.096
|
?70.417
|
|
|
|
160
|
2.34
|
?69.682
|
|
|
|
161
|
18.91
|
?68.065
|
|
|
|
162
|
25.51
|
?68.190
|
|
|
|
163
|
24.90
|
?66.390
|
|
|
|
164
|
28.19
|
?65.977
|
holmium
|
67
|
Ho
|
165
|
100
|
?64.907
|
erbium
|
68
|
Er
|
162
|
0.137
|
?66.346
|
|
|
|
164
|
1.609
|
?65.953
|
|
|
|
166
|
33.61
|
?64.934
|
|
|
|
167
|
22.93
|
?63.299
|
|
|
|
168
|
26.79
|
?62.999
|
|
|
|
170
|
14.93
|
?60.118
|
thulium
|
69
|
Tm
|
169
|
100
|
?61.282
|
ytterbium
|
70
|
Yb
|
168
|
0.127
|
?61.577
|
|
|
|
170
|
3.04
|
?60.772
|
|
|
|
171
|
14.28
|
?59.315
|
|
|
|
172
|
21.83
|
?59.264
|
|
|
|
173
|
16.13
|
?57.560
|
|
|
|
174
|
31.83
|
?56.953
|
|
|
|
176
|
12.76
|
?53.497
|
lutetium
|
71
|
Lu
|
175
|
97.416
|
?55.174
|
|
|
|
176
|
2.584
|
?53.391
|
hafnium
|
72
|
Hf
|
174
|
0.1620
|
?55.852
|
|
|
|
176
|
5.604
|
?54.584
|
|
|
|
177
|
18.5953
|
?52.890
|
|
|
|
178
|
27.811
|
?52.445
|
|
|
|
179
|
13.6210
|
?50.473
|
|
|
|
180
|
35.0802
|
?49.790
|
tantalum
|
73
|
Ta
|
180
|
0.0123
|
?48.935
|
|
|
|
181
|
99.9877
|
?48.441
|
tungsten
|
74
|
W
|
180
|
0.1198
|
?49.643
|
|
|
|
182
|
26.4985
|
?48.246
|
|
|
|
183
|
14.3136
|
?46.366
|
|
|
|
184
|
30.6422
|
?45.706
|
|
|
|
186
|
28.4259
|
?42.511
|
rhenium
|
75
|
Re
|
185
|
37.398
|
?43.821
|
|
|
|
187
|
62.602
|
?41.218
|
osmium
|
76
|
Os
|
184
|
0.0197
|
?44.254
|
|
|
|
186
|
1.5859
|
?42.999
|
|
|
|
187
|
1.9644
|
?41.220
|
|
|
|
188
|
13.2434
|
?41.138
|
|
|
|
189
|
16.1466
|
?38.988
|
|
|
|
190
|
26.2584
|
?38.708
|
|
|
|
192
|
40.7815
|
?35.882
|
iridium
|
77
|
Ir
|
191
|
37.272
|
?36.709
|
|
|
|
193
|
62.728
|
?34.536
|
platinum
|
78
|
Pt
|
190
|
0.013634
|
?37.325
|
|
|
|
192
|
0.782659
|
?36.296
|
|
|
|
194
|
32.96700
|
?34.779
|
|
|
|
195
|
33.831557
|
?32.812
|
|
|
|
196
|
25.24166
|
?32.663
|
|
|
|
198
|
7.16349
|
?29.923
|
gold
|
79
|
Au
|
197
|
100
|
?31.157
|
mercury
|
80
|
Hg
|
196
|
0.15344
|
?31.843
|
|
|
|
198
|
9.968
|
?30.970
|
|
|
|
199
|
16.873
|
?29.563
|
|
|
|
200
|
23.096
|
?29.520
|
|
|
|
201
|
13.181
|
?27.679
|
|
|
|
202
|
29.863
|
?27.362
|
|
|
|
204
|
6.865
|
?24.707
|
thallium
|
81
|
Tl
|
203
|
29.524
|
?25.775
|
|
|
|
205
|
70.476
|
?23.834
|
lead
|
82
|
Pb
|
204
|
1.4245
|
?25.123
|
|
|
|
206
|
24.1447
|
?23.801
|
|
|
|
207
|
22.0827
|
?22.467
|
|
|
|
208
|
52.3481
|
?21.764
|
bismuth
|
83
|
Bi
|
209
|
100
|
?18.273
|
thorium
|
90
|
Th
|
232
|
100
|
35.444
|
uranium
|
92
|
U
|
234
|
0.00548
|
38.141
|
|
|
|
235
|
0.7200
|
40.914
|
|
|
|
238
|
99.2745
|
47.304
|
The specification of
Z
,
A
, and the
chemical symbol
(a one- or two-letter abbreviation of the element’s name, say Sy) in the form
A
Z
Sy identifies an isotope adequately for most purposes. Thus, in the standard notation,
1
1
H refers to the simplest isotope of hydrogen and
235
92
U to an isotope of uranium widely used for
nuclear power
generation and nuclear weapons fabrication. (Authors who do not wish to use symbols sometimes write out the element name and mass number?hydrogen-1 and
uranium-235
in the examples above.)
The term
nuclide
is used to describe particular isotopes, notably in cases where the nuclear rather than the chemical properties of an atom are to be
emphasized
. The lexicon of isotopes includes three other frequently used terms:
isotones
for isotopes of different elements with the same number of neutrons,
isobars
for isotopes of different elements with the same mass number, and
isomers
for isotopes identical in all respects except for the total energy content of the nuclei.
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