Isotopes vs. Isobars: What's the Difference?

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Isotopes are atoms of the same element with the same atomic number but different mass numbers, while isobars are atoms of different elements that have the same mass number. Isotopes differ only in their number of neutrons, so they behave alike chemically; isobars have different numbers of protons, so they are different elements with different chemical properties.

At a glance

	Isotopes	Isobars
Atomic number (Z)	Same	Different
Mass number (A)	Different	Same
Element	Same element	Different elements
Chemical properties	Nearly identical	Different
Example	Carbon-12 and carbon-14	Argon-40 and calcium-40

Which should you use?

Isotopes

Isotopes matter whenever the neutron count changes an atom's stability or mass, such as carbon-14 in radiocarbon dating or uranium-235 as fuel in nuclear power stations.

Isobars

Isobars describe atoms of different elements that weigh nearly the same, such as argon-40 (18 protons) and calcium-40 (20 protons), both with a mass number of 40.

Frequently asked questions

Why do isotopes have the same chemical properties?

Chemical behaviour depends on electrons, and the number of electrons in a neutral atom equals its number of protons. Isotopes have the same number of protons and electrons, so they react alike. Only physical properties linked to mass, such as density, differ slightly.

What are the isotopes of hydrogen?

Hydrogen has three isotopes: protium with no neutrons, deuterium with one neutron and tritium with two. All have one proton and one electron. Protium is by far the most common, deuterium forms heavy water, and tritium is radioactive.

What are isotones?

Isotones are atoms of different elements with the same number of neutrons but different numbers of protons. For example, carbon-14 and nitrogen-15 each contain 8 neutrons. They complete the set with isotopes (same protons) and isobars (same mass number).