Ionisation - GCSE Physics Definition

What is ionisation?

In GCSE physics, ionisation is the process of adding or removing an electron from an atom to form an ion.

An atom consists of a positively charged nucleus, containing protons and neutrons, surrounded by negatively charged electrons. The number of protons in the nucleus is equal to the number of orbiting electrons, meaning atoms are electrically neutral, or uncharged.

If the number of protons or electrons changes in any way, then the atom will become charged and turn into an ion. Protons are too tightly bound within a nucleus, so they cannot be removed. Therefore, the only way an atom's charge can change is by gaining or losing electrons.

• If an atom loses an electron, it forms a positive ion

• If an atom gains an electron, it forms a negative ion

Positive ions often form when radioactive emissions, such as alpha, beta, or gamma radiation, pass nearby or collide with an atom, knocking out one or more of its outer electrons. This is why these particles are often called ionising radiation, as this is the property that makes radioactive emissions so dangerous.