Ionic, Covalent and Metallic Bonding

This guide covers subtopics 2.4 to 2.7 for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Bonding is where most marks are won and lost, because nearly every question asks you to link structure and bonding to properties. Memorising properties without the explanation will cap your marks.

A note on labels: 0620 separates Core and Extended; 5070 does not, and everything listed for 5070 is required. In this topic the split matters a great deal — several explanations that are Extended-only at IGCSE are compulsory at O Level.

2.4 Ions and ionic bonds

0620 calls this “Ions and ionic bonds”; 5070 calls it “Ion and ionic bonds”. Same content area, different wording in the two syllabuses.

Atoms form ions to achieve a full outer shell. Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions).

An ionic bond is a strong electrostatic attraction between oppositely charged ions.

CORE (0620) · REQUIRED (5070) — formation of cations and anions; the definition of an ionic bond; formation of ionic bonds between Group I and Group VII elements; using dot-and-cross diagrams to show that formation by electron transfer; properties of ionic compounds.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions; formation of ionic bonds between metallic and non-metallic elements; and explain the properties of ionic compounds in terms of structure and bonding.

That last point is the crucial difference. At O Level you must explain the properties. At IGCSE Core you need only describe them.

Explaining the properties

Ionic compounds have high melting and boiling points because a large amount of energy is needed to overcome the many strong electrostatic attractions throughout the giant lattice.

They conduct electricity when molten or in aqueous solution but not when solid, because the ions are only free to move in the liquid or dissolved state. In the solid the ions are held in fixed positions.

Notice that both explanations refer to the lattice. That is why the giant lattice outcome and the explanation outcome sit together.

Dot-and-cross diagrams for ionic bonding

A dot-and-cross diagram shows only the outer shell electrons, using dots for the electrons of one atom and crosses for the electrons of the other, so you can see which atom each electron originally came from. Square brackets and the ion’s charge go around each ion.

Sodium chloride, NaCl — sodium (2,8,1) transfers its single outer electron to chlorine (2,8,7):

             ××                             ××
 Na•   +   ××Cl×      →      [Na]⁺      [ ××Clו ]⁻
             ××                             ××

Na: 1 outer electron (•), lost      Cl⁻: 7 crosses (its own) + 1 dot (from Na)
[Na]⁺: no outer electrons left      = 8 outer electrons, a full outer shell

Magnesium oxide, MgO (0620 Extended, 5070 required — 0620 Core covers only Group I with Group VII) — magnesium (2,8,2) transfers both outer electrons to oxygen (2,6), so the ratio is 1:1 and each ion carries a 2+ or 2− charge:

              ××                             ××
 •Mg•   +   ××O       →      [Mg]²⁺      [ ××O•• ]²⁻
              ××                             ××

Mg: 2 outer electrons (• •), lost   O²⁻: 6 crosses (its own) + 2 dots (from Mg)
[Mg]²⁺: no outer electrons left     = 8 outer electrons, a full outer shell

In both cases the metal ion has lost its whole outer shell, so no dots are left around it (if a shell is drawn for the ion, it is the full shell underneath, now the outer one), and the non-metal ion has 8 outer electrons: its own electrons as crosses plus the transferred electrons as dots. Count the symbols inside each bracket before moving on.

2.5 Simple molecules and covalent bonds

A covalent bond forms when a pair of electrons is shared between two atoms, leading to a noble gas electronic configuration for both.

CORE (0620) · REQUIRED (5070) — the definition; formation of covalent bonds in simple molecules; using dot-and-cross diagrams to show single covalent bonds (e.g. water, methane, hydrogen chloride); and describing the properties of simple molecular compounds.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — covalent bonding in a wider range of molecules, including dot-and-cross diagrams for molecules with double bonds such as carbon dioxide, and explaining the properties of simple molecular compounds in terms of structure and bonding.

Simple molecular substances have low melting and boiling points because, although the covalent bonds within each molecule are strong, the forces between molecules are weak and little energy is needed to separate them. They generally do not conduct electricity because there are no free electrons or ions.

Dot-and-cross diagrams for covalent bonding

Here only the electrons in the bond need to be shared to give both atoms a full outer shell; dots and crosses again mark which atom each electron came from, and a shared pair sits where the outer shells overlap.

Water, H₂O — oxygen (2,6) shares one electron with each of two hydrogen atoms:

       ××
   H •× O ו H
       ××

O: 6 crosses (4 in two lone pairs, 1 in each bond); each H: 1 dot
O ends with 8 outer electrons, each H with 2 (a full shell for hydrogen)
(the diagram shows how electrons are shared, not the bent shape of the molecule)

Methane, CH₄ — carbon (2,4) shares one electron with each of four hydrogen atoms, giving carbon 8 outer electrons arranged around it and each hydrogen a full shell of 2.

Hydrogen chloride, HCl — hydrogen and chlorine share one pair of electrons; hydrogen ends with 2 outer electrons, chlorine ends with 8.

Carbon dioxide, CO₂ (Extended/Supplement — double bonds) — carbon (2,4) forms two shared pairs (a double bond) with each oxygen atom, so carbon has 8 outer electrons (4 shared pairs in total) and each oxygen has 8 (2 of its own lone pairs plus 2 shared pairs from the double bond).

The single most common error in this topic is confusing these two. Melting a simple molecular substance breaks intermolecular forces, not covalent bonds.

2.6 Giant covalent structures

CORE (0620) · REQUIRED (5070) — describe the giant covalent structures of graphite and diamond, and relate their structures and bonding to their uses.

EXTENDED / SUPPLEMENT (0620) — the giant covalent structure of silicon(IV) oxide, SiO₂, and the similarity in properties between diamond and silicon(IV) oxide. 5070 lists silicon(IV) oxide within its required content, so O Level candidates must cover it.

In diamond, each carbon atom forms four covalent bonds in a rigid three-dimensional structure, making it very hard and giving it a very high melting point. It is used in cutting tools.

In graphite, each carbon atom forms three covalent bonds, producing layers held together by weak forces. The layers slide over one another, so graphite is soft and slippery and is used as a lubricant. Each carbon atom has one delocalised electron, so graphite conducts electricity — unusually for a non-metal.

Both are forms of carbon. The difference in properties comes entirely from the difference in structure and bonding, which is exactly what examiners want you to say.

2.7 Metallic bonding

EXTENDED / SUPPLEMENT (0620) — the whole of subtopic 2.7. The Core column for 2.7 is empty, so metallic bonding is not required for IGCSE Core candidates at all.

REQUIRED (5070) — 5070 has no tiering, so both outcomes are required at O Level.

This is the sharpest Core/Extended difference in the topic. If you are an IGCSE Core candidate, you are not required to study metallic bonding. If you are IGCSE Extended or an O Level candidate, you are.

Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a “sea” of delocalised electrons.

This explains why metals conduct electricity — the delocalised electrons are free to move — and why they are malleable and ductile: layers of ions can slide over one another without breaking the bonding.

Common mistakes

• Saying ionic compounds conduct when solid. They do not — the ions cannot move.
• Saying covalent bonds break when a simple molecular substance melts. Intermolecular forces break, not the covalent bonds.
• Describing when the question says explain. Structure-and-bonding questions almost always want the explanation.
• Forgetting that graphite conducts because of its delocalised electrons.
• IGCSE Core candidates assuming they must explain ionic properties — or O Level candidates assuming they need not.

Quick revision checklist

• Formation of cations and anions; definition of an ionic bond
• Dot-and-cross diagrams for ionic bonding between Group I and Group VII elements (e.g. sodium chloride)
• (0620 Extended, 5070 required) dot-and-cross diagrams for other metal–non-metal ionic compounds (e.g. magnesium oxide)
• Properties of ionic compounds
• (0620 Extended, 5070 required) giant ionic lattice, and explaining the properties
• Covalent bond defined; bonding in simple molecules
• Dot-and-cross diagrams for single covalent bonds (e.g. water, methane, hydrogen chloride)
• (0620 Extended, 5070 required) dot-and-cross diagrams for molecules with double bonds (e.g. carbon dioxide)
• (0620 Extended, 5070 required) explaining properties of simple molecular compounds
• Diamond and graphite: structure, bonding, properties, uses
• (0620 Extended, 5070 required) silicon(IV) oxide
• (0620 Extended only — not required for Core; 5070 required) metallic bonding, and how it explains conductivity and malleability

Related resources

• Atomic Structure — electronic configuration first
• States of Matter and Kinetic Particle Theory
• Formulae, Equations and the Mole
• Identification of Ions and Gases — the ionic charges used in test reagents
• The Periodic Table: Groups and Trends — why group number sets ionic charge
• Metal Properties and Reactivity — metallic bonding’s physical consequences, and the reactivity series
• Chemical Bonding: Electronegativity, Ionic and Metallic Bonds — the AS Level continuation of ionic and metallic bonding (9701)
• Chemical Bonding: Shapes and Intermolecular Forces — the AS Level continuation of covalent bonding (9701)
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.