4.1.3 Atomic & Mass Number

Mass Number, Atomic Number and Isotopes

All atoms of the same element have the same number of protons. The number of protons is called the atomic number.

In a normal atom, the number of electrons is equal to the number of protons and so the atomic number also tells us how many electrons are in the atom.

The mass number is the number of neutrons plus the number of protons. Atoms of the same element may have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. So different isotopes have different mass numbers but the same proton number.

infoNote

If we know the number mass number and the atomic number, we can calculate the number of neutrons in the atom using:

number of neutrons=mass number−atomic number

infoNote

Atoms may also lose or gain electrons. This process is called ionisation and the atoms become ions. These ions are positively charged if they have lost electrons or negatively charged if they have gained electrons.

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