Foundation knowledge: Isotopes and relative atomic mass

Understanding the nature of isotopes and how to calculate relative atomic mass.

Isotopes

At a glance:

Isotopes are atoms of the same element with different numbers of neutrons. Isotopes tend to have the same chemical properties, as they have the same number of electrons, but can have different physical properties because they have different mass numbers.

Atoms contain protons and neutrons bound together in the nucleus. Protons and neutrons both have a relative mass of one unit.

Although atoms of the same element always have the same number of protons, they can have different numbers of neutrons. For example, carbon atoms always have 6 protons , but can have 6, 7 or 8 neutrons. This means that the mass number of these isotopes will be 12, 13 and 14 respectively.

Carbon-12 is the most common naturally occuring isotope for carbon; only about 1% of carbon is carbon-13 and even less is carbon-14, which has an unstable nucleus and is radioactive.

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