Ions

Section 1 of 7What is an ion?

Charged particle

• Charged particle An ion is an atom or group of atoms with an overall electrical charge, because it has gained or lost electrons.

Why ions form

• Why ions form Atoms gain a more stable electronic structure by having a full outer shell, like a noble gas.

Number of protons

• Number of protons The number of protons never changes when an ion forms. Only the electrons change.

Section 2 of 7Metals and non-metals

1Metal atom

• Metal atom Metals in groups 1, 2 and 3 have few outer electrons, so they lose them.

2Positive ion

• Positive ion Losing electrons leaves more protons than electrons, forming a cation. Na → Na⁺ + e⁻.

3Non-metal atom

• Non-metal atom Non-metals in groups 5, 6 and 7 have nearly full outer shells, so they gain electrons.

4Negative ion

• Negative ion Gaining electrons gives an anion. Cl + e⁻ → Cl⁻.

Section 3 of 7Predicting ion charges

Group	Change	Ion charge	Example
1	Lose 1 electron	1+	Li+, Na+, K+
2	Lose 2 electrons	2+	Mg2+, Ca2+
3	Lose 3 electrons	3+	Al3+
5	Gain 3 electrons	3−	N3−
6	Gain 2 electrons	2−	O2−, S2−
7	Gain 1 electron	1−	F−, Cl−, Br−

Section 4 of 7Working out an ion

1

Write the electronic configuration of the atom, for example magnesium 2,8,2.

2

Decide whether it is easier to lose or gain electrons to reach a full outer shell. Magnesium loses 2.

3

The ion's configuration is 2,8, the same as neon.

4

Charge = protons − electrons = 12 − 10 = 2+, so the ion is Mg²⁺.

Section 5 of 7Compound ions

Group ions

• Group ions Some ions contain several atoms, such as hydroxide OH⁻, nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻ and ammonium NH₄⁺.

Metals in the middle

• Metals in the middle Transition metals can form ions with different charges, such as Fe²⁺ and Fe³⁺.

Section 6 of 7Key terms

Term	Meaning
Ion	An atom or group of atoms with a positive or negative charge.
Cation	A positively charged ion, formed when atoms lose electrons.
Anion	A negatively charged ion, formed when atoms gain electrons.
Noble gas structure	A full outer shell of electrons, which is very stable.
Electron	A negatively charged particle that orbits the nucleus.

Section 7 of 7Exam technique

1

Saying that metals gain electrons to form positive ions. They lose electrons.

2

Thinking that protons are transferred. Only electrons move in ion formation.

3

Writing the charge as Mg² or Mg+2. It is Mg²⁺.

4

Forgetting that a chloride ion has 18 electrons, not 17.