Ions

Ions are positively or negatively charged particles that are formed when an atom or group of atoms gains or loses electrons.

Due to the fact that ions are formed when atoms and groups of atoms either gain or lose electrons this means that ions do not have equal numbers of protons and electrons.

Ions are represented by a symbol which shows the atom or groups of atoms and the positive or negative charge of the ion.

Monatomic ions

Monatomic ions are the most simple type of ions that form when one atom loses or gains one or more electron(s). Hence the prefix 'mono' which means 'one'.

Metal atoms (such as sodium, calcium, magnesium, & aluminium), form the positively charge ions (or cations) Na⁺, Ca²⁺, Mg²⁺ and Al³⁺ when they lose one or more valence shell electron(s).

Examples

Na (2,8,1) → Na⁺ (2,8) the sodium atom loses its valence electron to become the positively charged sodium ion.

Ca (2,8,8,2) → Ca²⁺ (2,8,8) the calcium atom loses its two valence electrons to become the positively charged calcium ion.

Mg (2,8,2) → Mg²⁺ (2,8) the magnesium atom loses its two valence electrons to become the positively charged magnesium ion.

Al (2,8,3) → Al³⁺ (2,8) the aluminium atom loses its three valence electrons to become the positively charged aluminium ion.

These can be represented by diagrams as well.

Non-metal atoms (such as oxygen, sulfur, chlorine, nitrogen) form the negatively charged ions (or anions) O^2-, S^2-, Cl^- and N^3- when they gain one or more valence electron(s).

Examples

O (2,6) → O^2- (2,8) the oxygen atom gains two valence electrons to become the negatively charged oxide ion.

S (2,8,6) → S^2- (2,8,8) the sulfur atom gains two valence electrons to become the negatively charged sulfide ion.

Cl (2,8,7) → Cl^- (2,8) the chlorine atom gains one valence electron to become the negatively charged chloride ion.

N (2,5) → N^3- (2,8) the nitrogen atom gains three valence electrons to become the negatively charged nitride ion.

These can be represented by diagrams as well.

Video 1 - What are ions?

Video 2 - Atoms and ions

Names of monatomic ions

Polyatomic ions

Polyatomic ions are ions that are made up two or more different types of atom. Sometimes they are called compound ions.

The polyatomic ions that you need to know for NCEA Level 1 are given on the ion table (provided on a resource sheet).

NH₄⁺ - the ammonium ion, is a polyatomic ion made up of five atoms (one nitrogen atom, N, and 4 hydrogen atoms, H₄ ).