How is an ion formed?

Under certain conditions, a neutral atom can gain or lose electrons. This new entity, which has become electrically charged, is called an ion.

What moves, what doesn’t

It is only the electrons that leave or arrive: the nucleus itself never changes during this process. The number of protons remains the same, so the chemical element remains the same. A sodium atom that loses an electron remains a sodium atom — simply a charged one.

Two types of ions

Name	What happens	Resulting charge
Cation	the atom loses one or more electrons	positive
Anion	the atom gains one or more electrons	negative

Losing an electron leaves one ‘extra’ proton in the charge balance: the charge therefore becomes positive. Gaining an electron adds an extra negative charge: the charge becomes negative.

The mistake you must avoid at all costs

‘An ion is a broken or damaged atom. ’ This idea is false and misleading. An ion is not damaged: it is a perfectly stable and common structure, often more stable than the original neutral atom. The table salt you use every day is made up entirely of ions (sodium and chloride), not neutral sodium and chlorine atoms — in fact, a pure sodium atom reacts violently on contact with water, whereas the sodium ion, present in dissolved salt, poses no danger whatsoever. Becoming an ion changes the chemical properties of a species; it is not a form of degradation.