2.4Atoms, Elements and Compounds

Ions and ionic bonds — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 7 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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Lesson summary

2.4 Ions and ionic bonds — Notes — IGCSE Chemistry notes: Describe ion formation, ionic bonding between metals and non-metals, the ionic lattice and the properties of ionic compounds.Ion: an electrically charged atom or group of atoms formed by the loss or gain of electrons.Atoms…

Electrostatic attraction
Atoms lose or gain electrons to obtain a full outer shell
Mg (2,8,2) loses 2 electrons to form Mg2+ (2,8)
O (2,6) gains 2 electrons to form O2− (2,8)
All metals lose electrons to form positive ions and all non-metals gain electrons to form negative ions
Compound ions: OH−, NO3−, SO42−, CO32−, NH4+

Part 1 of 7Formation of ions

Ion: an electrically charged atom or group of atoms formed by the loss or gain of electrons.

Atoms lose or gain electrons to obtain a full outer shell.

Ions of Groups I, II, VI and VII have the electronic structure of a noble gas, such as helium, neon or argon.

Na⁺ and F⁻ both have 2,8 like neon.

Cations: positive ions formed when an atom loses electrons, so they have more protons than electrons.

Na (2,8,1) → Na⁺ (2,8) + e⁻.
Mg (2,8,2) loses 2 electrons to form Mg²⁺ (2,8).

Anions: negative ions formed when an atom gains electrons, so they have more electrons than protons.

Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8).
O (2,6) gains 2 electrons to form O²⁻ (2,8).

All metals lose electrons to form positive ions and all non-metals gain electrons to form negative ions.

Part 2 of 7Charges of ions from the group

Group	Electrons lost or gained	Ion charge	Example
I	Lose 1	1+	Na⁺, K⁺
II	Lose 2	2+	Mg²⁺, Ca²⁺
III	Lose 3	3+	Al³⁺
V	Gain 3	3−	N³⁻
VI	Gain 2	2−	O²⁻, S²⁻
VII	Gain 1	1−	F⁻, Cl⁻

Compound ionsOH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, NH₄⁺.

Part 3 of 7Ionic bonding

Ionic compounds form when metal atoms react with non-metal atoms.

Electrons are transferred, not shared: metal atoms lose their outer electrons and the non-metal atoms gain them to form positive and negative ions.

Ionic bond: the strong electrostatic force of attraction between oppositely charged ions that holds the compound together.

Formula rule: the total positive charge equals the total negative charge,

so the compound has no overall charge and more than one ion may be needed (Ca²⁺ + 2Cl⁻ gives CaCl₂).

Sodium chloride

A sodium atom (2,8,1) transfers its one outer electron to a chlorine atom (2,8,7).
Sodium becomes Na⁺ (2,8) and chlorine becomes Cl⁻ (2,8,8).
The oppositely charged ions attract strongly, which is the ionic bond.
Many ions arrange into a giant lattice of alternating Na⁺ and Cl⁻.

Other examples

MgCl₂: Mg gives one electron to each of two Cl atoms.
Na₂O: two Na atoms each give one electron to one O atom.
MgOtwo electrons transfer from Mg to O.

Part 4 of 7Dot-and-cross diagrams of ionic compounds

What they show: the arrangement of the outer-shell electrons, usually only the outer shell (all shells if asked).

Method

Write the electronic configurations of the metal and non-metal atoms.
Work out how many electrons the metal must lose and the non-metal must gain.
Balance the numbers so electrons lost equal electrons gained, which gives the formula.
Draw the ions with outer shellscrosses for the metal's transferred electrons and dots for the non-metal's.
Put square brackets round each ion and write the charge at the top right, outside the bracket.

Sodium chloride: sodium (Group I) loses one outer electron to form Na⁺, chlorine (Group VII) gains one electron to form Cl⁻.

Magnesium oxide (Supplement): magnesium loses two electrons to form Mg²⁺, oxygen gains two to form O²⁻, giving MgO with no overall charge.

Any combination (Supplement): the method extends to AlF₃, CaO and Li₃N.

Part 5 of 7Giant ionic lattice (Supplement)

Giant ionic lattice (Supplement)

Lattice: a regular, repeating three-dimensional arrangement of alternating positive and negative ions held by strong electrostatic attraction in all directions.

NaCl is cubic, with each Na⁺ surrounded by 6 Cl⁻ and each Cl⁻ by 6 Na⁺.

Sodium chloride is a giant lattice, so it has no molecules.

Part 6 of 7Properties of ionic compounds

Property	Explanation
High melting and boiling points, usually solid at room temperature	Strong electrostatic attraction between oppositely charged ions throughout the giant lattice needs a lot of energy to overcome
Do not conduct when solid	Ions are held in fixed positions and cannot move
Conduct when molten or aqueous	Ions are free to move and carry charge
Often soluble in water	Water molecules attract and separate the ions

Properties of ionic compounds

Many ionic compounds are insoluble in organic solvents such as hexane.

Electrons do not move through an ionic compound, as ions carry the charge.

Effect of charge (Supplement): the greater the ion charges, the stronger the attraction and the higher the melting point.

MgO (Mg²⁺, O²⁻) melts at about 2850 °C, but NaCl (Na⁺, Cl⁻) at about 800 °C.

Part 7 of 7More from the specification

1Lattice cohesion and electrostatic forces

Lattice cohesion and electrostatic forces omnidirectional electrostatic forces attract oppositely charged cations and anions tightly within three-dimensional crystalline ionic lattices.

2Element

A substance made of only one type of atom.

Key terms

Term	Meaning
Ion	a charged atom or group of atoms formed by the loss or gain of electrons.
Cation	a positive ion, formed when an atom loses electrons.
Anion	a negative ion, formed when an atom gains electrons.
Ionic bond	the strong electrostatic attraction between oppositely charged ions.
Lattice	a regular, repeating three-dimensional arrangement of particles.
Electrostatic attraction	the force between opposite charges.

Avoid these mistakes

Saying ionic bonds are shared electronselectrons are transferred and the bond is the attraction between ions.

Saying ionic solids conduct electricitythe ions cannot move until molten or dissolved.

Saying ionic compounds conduct because electrons move: ions are free to move when molten or aqueous.

Writing sodium chloride moleculesit has a giant ionic lattice with no molecules.

Forgetting the square brackets and charge on an ionic dot-and-cross diagram.

Last updated: 15 October 2026