isotope

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isotope summary

isotope

Q: What is an isotope?

An isotope is 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 behavior but with different atomic masses and physical properties. Every chemical element has one or more isotopes.

Q: Why do isotopes have different properties?

Differences in the properties of isotopes can be attributed to either of two causes: differences in mass or differences in nuclear structure. Scientists usually refer to the former as isotope effects and to the latter by a variety of more specialized names.

Q: When are isotopes stable?

Isotopes are said to be stable if, when left alone, they show no perceptible tendency to change spontaneously. A uniform scale of nuclear stability that applies to both stable and unstable isotopes alike is based on comparing measured isotope masses with the masses of their constituent electrons, protons, and neutrons.

Q: How were isotopes discovered?

The existence of isotopes emerged from two independent lines of research, the first being the study of radioactivity. The unambiguous confirmation of isotopes in stable elements not associated directly with either uranium or thorium came with the development of the mass spectrograph.

chemistry

Written by

Gregory F. Herzog

Fact-checked by

Britannica Editors

Last updated

Oct. 17, 2026 •History

What is an isotope?

Why do isotopes have different properties?

When are isotopes stable?

How were isotopes discovered?

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).

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_ZSy identifies an isotope adequately for most purposes. Thus, in the standard notation, ¹₁H refers to the simplest isotope of hydrogen and ²³⁵₉₂U 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.)

Periodic Table of the elements concept image (chemistry)

Britannica Quiz

Facts You Should Know: The Periodic Table Quiz

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.

isotope

What is an isotope?

Why do isotopes have different properties?

When are isotopes stable?

How were isotopes discovered?

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