Isotopes

Key Concepts

• Isotopes are atoms of the same element that have a different number of neutrons in their nuclei.

• Isotopes of an element will have the same Atomic Number (Z) but a different Mass Number (A):

  Atomic Number (Z) is the number of protons in the nucleus of an atom and is the same for all atoms of the same element.

  Mass Number (A) is the number of protons + the number of neutrons in the nucleus of an atom and will be different for each isotope of an element.

• A particular isotope of an element can be referred to in a number of ways:

  name-A, eg, carbon-12, carbon-14

  ^ASymbol, eg, ¹²C, ¹⁴C
  

  A Symbol Z

  eg,

  12 C, 6

  14 C, 6

• The number of neutrons in the nucleus of an atom can be calculated using the Atomic Number (Z) and the Mass Number (A): A - Z = number of neutrons.

• Isotopes can be classified as natural (found in nature) or man made (artificial or synthetic).

• Isotopes can also be classified as stable or unstable:

1. A stable isotope does NOT undergo radioactive (or nuclear) decay.

2. An unstable isotope undergoes radioactive (or nuclear) decay.
   Unstable isotopes are also known as radioisotopes or radionucleides

Examples of Naturally Occurring Isotopes

Element	Atomic Number (Z) (= no. protons)	Isotope	Mass Number (A) (= no. protons + no. neutrons)	Number of Neutrons (A - Z)
hydrogen	1	hydrogen-1, 1H, 11H	1	1 - 1 = 0
		hydrogen-2, 2H, 21H	2	2 - 1 = 1
helium	2	helium-3, 3He, 32He	3	3 - 2 = 1
		helium-4, 4He, 42He	4	4 - 2 = 2
lithium	3	lithium-6, 6Li, 63Li	3	6 - 3 = 3
		lithium-7, 7Li, 73Li	7	7 - 3 = 4
boron	5	boron-10, 10B, 105B	10	10 - 5 = 5
		boron-11, 11B, 115B	11	11 - 5 = 6
carbon	6	carbon-12, 12C, 126C	12	12 - 6 = 6
		carbon-13, 13C, 136C	13	13 - 6 = 7
		carbon-14, 14C, 146C	14	14 - 6 = 8
nitrogen	7	nitrogen-14, 14N, 147N	14	14 - 7 = 7
		nitrogen-15, 15N, 157N	15	15 - 7 = 8
oxygen	8	oxygen-16, 16O, 168O	16	16 - 8 = 8
		oxygen-17, 17O, 178O	17	17 - 8 = 9
		oxygen-18, 18O, 188O	18	18 - 8 = 10
neon	10	neon-20, 20Ne, 2010Ne	20	20 - 10 = 10
		neon-21, 21Ne, 2110Ne	21	21 - 10 = 11
		neon-22, 22Ne, 2210Ne	22	22 - 10 = 12

Examples of Naturally Occurring Stable and Unstable Isotopes (radioisotopes or radionucleides)

Element	Stable Isotope(s)	Radioisotope(s)
carbon	carbon-12	carbon-14
potassium	potassium-39 potassium-41	potassium-40
calcium	calcium-40 calcium-42 calcium-43 calcium-44 calcium-46	calcium-48
vanadium	vanadium-51	vanadium-50
germanium	germanium-70 germanium-72 germanium-73 germanium-74	germanium-76
selenium	selenium-74 selenium-74 selenium-76 selenium-77 selenium-78 selenium-80	selenium-82
krypton	krypton-80 krypton-82 krypton-83 krypton-84 krypton-86	krypton-78
rubidium	rubidium-85	rubidium-87
cadmium	cadmium-106 cadmium-108 cadmium-110 cadmium-111 cadmium-112 cadmium-114	cadmium-113 cadmium-116
osmium	osmium-184 osmium-187 osmium-188 osmium-189 osmium-190 osmium-192	osmium-186
platinum	platinum-192 platinum-194 platinum-195 platinum-196 platinum-198	platinum-190
uranium		uranium-234 uranium-235 uranium-238

Examples of Naturally Occurring and Man Made Isotopes

Element	Naturally Occurring Isotope(s)	Man Made Radioisotope(s)
hydrogen	hydrogen-1 hydrogen-2	hydrogen-3
sodium	sodium-23	sodium-22
phosphorus	phosphorus-31	phosphorus-32 phosphorus-33
chlorine	chlorine-35 chlorine-37	chlorine-36
technetium		technetium-95 technetium-96 technetium-97 technetium-98 technetium-99
iodine	iodine-127	iodine-124 iodine-125 iodine-126 iodine-129 iodine-131
lead	lead-204 lead-206 lead-207 lead-208	lead-202 lead-203 lead-205 lead-210

All transuranic elements (those with Atomic Number greater than 92) are man made and unstable.

Examples of transuranic elements:

• neptunium, ²³⁴Np, ²³⁵Np, ²³⁶Np, ²³⁷Np, ²³⁸Np, ²³⁹Np

• plutonium, ²³⁶Pu, ²³⁷Pu, ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, ²⁴⁴Pu, ²⁴⁶Pu, ²⁴⁷Pu

• americium, ²⁴⁰Am, ²⁴¹Am, ²⁴²Am, ²⁴³Am

• curium, ²⁴⁰Cm, ²⁴¹Cm, ²⁴²Cm, ²⁴³Cm, ²⁴⁴Cm, ²⁴⁵Cm, ²⁴⁶Cm, ²⁴⁷Cm, ²⁴⁸Cm, ²⁵⁰Cm

• berkelium, ²⁴⁵Bk, ²⁴⁶Bk, ²⁴⁷Bk,²⁴⁸Bk, ²⁴⁹Bk

• californium, ²⁴⁶Cf, ²⁴⁸Cf, ²⁴⁹Cf, ²⁵⁰Cf, ²⁵¹Cf, ²⁵²Cf, ²⁵³Cf, ²⁵⁴Cf

• einsteinium, ²⁵¹Es, ²⁵²Es, ²⁵³Es, ²⁵⁴Es, ²⁵⁵Es

• fermium, ²⁵²Fm, ²⁵³Fm, ²⁵⁷Fm

• mendelevium, ²⁵⁸Md, ²⁶⁰Md

Search this Site

Become an AUS-e-TUTE Member

Subscribe to our Free Newsletter

AUS-e-NEWS is emailed out in
December, March, June, and September.

Ask Chris, the Chemist, a Question

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

Related AUS-e-TUTE Topics

Relative Atomic Mass

Mass Spectroscopy

Properties and Uses of Radiation and Isotopes

Uses of Carbon-14 for Dating

Uses of radioisotopes in Smoke Detectors

Nuclear Decay

Half-Life Definitions and Calculations

Bookmark AUS-e-TUTE