Covalent BondingOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is covalent bonding?
Covalent bonding happens between non-metal atoms. Instead of transferring electrons like in ionic bonding, the atoms share pairs of electrons so that each atom achieves the electron arrangement of a noble gas (a full outer shell).
- Each shared pair of electrons is one covalent bond
- Covalent bonds are strong because both nuclei are attracted to the shared pair of electrons
- Atoms can share more than one pair to form double or triple bonds
Examiners want the word 'shared pair of electrons' explicitly — 'sharing electrons' alone loses marks on some mark schemes.
Section 2
Which simple molecules form by covalent bonding?
You must be able to name and recognise these simple molecules formed by covalent bonding:
- H2 (hydrogen)
- Cl2 (chlorine)
- O2 (oxygen)
- N2 (nitrogen)
- HCl (hydrogen chloride)
- H2O (water)
- NH3 (ammonia)
- CH4 (methane)
Each of these is held together by covalent bonds between the atoms within the molecule.
Section 3
How do we represent covalent bonds?
Covalent bonding can be shown using two main types of diagram:
- Dot-and-cross diagrams — show the outer-shell electrons of each atom as dots or crosses, with shared pairs drawn between the atoms so it is clear which electrons came from which atom
- Displayed formulae — show each atom and use a single line to represent each covalent bond (e.g. H–H)
You should be able to draw or complete either type of diagram to show how elements share electrons to form covalent compounds.
Students often forget that only the outer (highest occupied) shell electrons are drawn in a dot-and-cross diagram — inner shells are left out.
Section 4
Simple molecules vs giant covalent structures
Covalent bonding does not only make small molecules — it can also make giant covalent structures, where huge numbers of atoms are linked by covalent bonds throughout the whole structure (e.g. diamond, graphite, silicon dioxide).
| Feature | Simple molecule | Giant covalent structure |
|---|---|---|
| Size | Few atoms | Millions of atoms |
| Bonds between molecules | Weak intermolecular forces | Not applicable — one giant structure |
| Example | H2O | Diamond |
You should be able to recognise which type a substance is from a diagram of its bonding.
Must Know
- Covalent bonding is the sharing of pairs of electrons between non-metal atoms
- Know the 8 named simple molecules: H2, Cl2, O2, N2, HCl, H2O, NH3, CH4
- Dot-and-cross diagrams show only outer-shell electrons and which atom they came from
- Displayed formulae use one line per covalent bond
- Covalent bonding can form either simple molecules or giant covalent structures
- Be able to identify simple molecules vs giant covalent structures from bonding diagrams
That's the notes covered.
Carry on to the next subtopic.