Isotopes vs Isobars vs Isotones

Every nucleus is defined by two numbers: how many protons it has (Z) and how many neutrons (N). Their sum is the mass number (A = Z + N). Pick two different nuclei and ask which of these three numbers they have in common, and you get three families with confusingly similar names:

• Isotopes have the same number of protons.
• Isobars have the same mass number.
• Isotones have the same number of neutrons.

This guide compares them side by side, gives a memory trick, and shows where each idea turns up in real chemistry.

The three definitions

Isotopes: same Z

Isotopes are nuclei with the same number of protons but different numbers of neutrons. Because the proton number defines the element, isotopes are always the same element.

Examples: ¹²C, ¹³C and ¹⁴C (all Z = 6); ³⁵Cl and ³⁷Cl (both Z = 17); ¹H, ²H and ³H.

Isotopes have the same chemistry because they have the same electrons, but different masses and nuclear stability. See isotopes, ions, isomers and allotropes.

Isobars: same A

Isobars are nuclei with the same mass number but different numbers of protons, which makes them different elements.

Examples: ⁴⁰Ar, ⁴⁰K and ⁴⁰Ca (all A = 40, with Z = 18, 19 and 20); ¹⁴C and ¹⁴N (both A = 14).

Isobars have roughly the same mass but completely different chemistry.

Isotones: same N

Isotones are nuclei with the same number of neutrons but different numbers of protons, so they are also different elements with different mass numbers.

Examples: ¹³C (6 p, 7 n) and ¹⁴N (7 p, 7 n); ³⁶S (16 p, 20 n), ³⁷Cl (17 p, 20 n), ³⁸Ar (18 p, 20 n), ³⁹K (19 p, 20 n) and ⁴⁰Ca (20 p, 20 n).

Comparison table

	Isotopes	Isobars	Isotones
Same	Protons (Z)	Mass number (A)	Neutrons (N)
Different	Neutrons, mass number	Protons, neutrons	Protons, mass number
Same element?	Yes	No	No
Same chemistry?	Yes (almost identical)	No	No
Similar mass?	Differ by whole units	Nearly the same	Differ
Example	³⁵Cl, ³⁷Cl	⁴⁰Ar, ⁴⁰Ca	¹³C, ¹⁴N

A memory trick

Look at one letter in each word:

• Isotope: same protons.
• Isobar: same A (mass number). (It also shares a root with “barometer”, from the Greek for heavy or weight.)
• Isotone: same neutrons.

Isotope and isotone differ by only one letter, and that letter (p or n) tells you which particle is shared.

Working out which is which: a method

Given two nuclei written as ᴬ_Z X:

1. Write down Z, A and N (= A − Z) for each.
2. Compare:
   • same Z → isotopes;
   • same A (different Z) → isobars;
   • same N (different Z) → isotones.
3. If none match, they’re unrelated in this sense. If all match, they’re the same nuclide (or one might be a nuclear isomer, an excited state of the same nucleus).

Example: compare ⁵⁶Fe (Z = 26) and ⁵⁸Ni (Z = 28).

• Fe: Z = 26, A = 56, N = 30.
• Ni: Z = 28, A = 58, N = 30. Same N → isotones.

Where these ideas matter

Isotopes: dating, tracing and medicine

Isotopes are everywhere in applied chemistry: carbon-14 dating, stable-isotope tracers in food authenticity and ecology, radioactive isotopes in medicine, and the isotope patterns that mass spectrometry uses to identify chlorine and bromine in molecules.

Isobars: beta decay and mass spectrometry clashes

Beta decay links isobars. In beta-minus decay, a neutron becomes a proton: Z goes up by 1 but A stays the same. So a beta decay always turns a nucleus into one of its isobars. Carbon-14 decays to nitrogen-14; potassium-40 decays to calcium-40 (by β⁻) or argon-40 (by electron capture). This last decay is the basis of potassium–argon dating of volcanic rocks.

Chains of beta decays move along a line of isobars towards the most stable one, which sits in the band of stability.

Isobaric interferences. In mass spectrometry, especially ICP-MS, isobars from different elements produce ions of almost identical mass, which can be mistaken for each other. Argon-40 from the plasma gas, for example, overlaps with calcium-40. Analysts avoid these clashes by measuring a different isotope, using collision cells, or with very high-resolution instruments that can separate the tiny mass differences between isobars.

Isotones: nuclear structure and magic numbers

Isotones are mostly used by nuclear physicists studying nuclear structure. Nuclei with a magic number of neutrons (such as N = 20, 28, 50, 82 or 126) are unusually stable, and comparing isotones along those lines shows how stability depends on neutrons separately from protons. For example, N = 82 has more stable isotones than any other neutron number. Isotones rarely appear in school chemistry except as a definition question.

Seeing the three families on a nuclide chart

Nuclear scientists organise every known nucleus on a chart of nuclides: a grid with the number of neutrons along the horizontal axis and the number of protons up the vertical axis. Each square is one nuclide. The three families then become three kinds of straight line through the chart, which is a helpful way to picture them.

• Isotopes lie along a horizontal row. Moving right along the row adds neutrons while the proton number, and therefore the element, stays fixed. Tin’s row is unusually long in the stable region, with ten stable isotopes.
• Isotones lie along a vertical column. Moving up the column adds protons while the neutron number stays fixed, so you pass from one element to the next.
• Isobars lie along a diagonal running from upper left to lower right. Each step down and to the right swaps a proton for a neutron, keeping the total the same. Beta decays move a nucleus one step along this diagonal, which is why decay chains of isobars look like staircases on the chart.

This picture also shows why only a narrow strip of the chart is stable. Along any isobar diagonal, the nuclei at the two ends have too many protons or too many neutrons. They decay towards the middle, where one or two stable isobars sit. For odd mass numbers there is usually just one stable isobar; for even mass numbers there can be two or three, which is one of the reasons even-mass nuclei are more varied.

The interactive element pages on this site list each element’s known isotopes, effectively one row of the chart at a time.

Related terms that cause confusion

• Isomers (chemical): molecules with the same formula but different structures, such as butane and methylpropane. Nothing to do with nuclei.
• Nuclear isomers: the same nuclide in a long-lived excited state, such as technetium-99m, widely used in medical imaging.
• Isoelectronic species: atoms or ions with the same number of electrons, such as Na⁺, Ne and F⁻ (see isoelectronic species).

Practice questions

Classify each pair as isotopes, isobars, isotones or none.

1. ²³⁵U and ²³⁸U
2. ³H and ³He
3. ³⁹K and ⁴⁰Ca
4. ¹⁴C and ¹⁶O
5. ²⁴Mg and ²⁶Mg
6. ¹⁴C and ¹⁶N

Answers:

1. Both Z = 92 → isotopes.
2. Both A = 3, Z = 1 and 2 → isobars. (Tritium beta-decays into helium-3.)
3. K: N = 20; Ca: N = 20 → isotones.
4. C: Z = 6, A = 14, N = 8; O: Z = 8, A = 16, N = 8 → same N → isotones.
5. Both Z = 12 → isotopes.
6. C: Z = 6, A = 14, N = 8; N: Z = 7, A = 16, N = 9 → none.

Key takeaways

• Isotopes: same protons, same element, same chemistry, different mass.
• Isobars: same mass number, different elements. Beta decay converts one isobar into another.
• Isotones: same number of neutrons, different elements, used mainly in nuclear structure studies.
• Remember: isotope = protons, isotone = neutrons, isobar = A.
• Check any element’s full list of isotopes on its page, for example potassium.