Structure and bonding · GCSE Chemistry
Bonding and structure
Ionic, covalent and metallic bonding, giant lattices versus simple molecules, and why diamond, graphite and metals behave as they do.
Ionic: transfer, giant lattice, high melting point. Covalent simple: share, weak between molecules, low melting point. Giant covalent or metallic: strong in all directions — and only delocalised electrons conduct.
The important bits
What you need to know
- 1
Ionic bonding is the electrostatic attraction between oppositely charged ions, formed when metals transfer electrons to non-metals. The structure is a giant ionic lattice.
- 2
Ionic compounds have high melting and boiling points because strong ionic bonds act in all directions. They conduct when molten or dissolved, because ions can move, but not when solid.
- 3
Covalent bonding is the sharing of electron pairs between non-metal atoms. Simple molecules such as H₂O, CO₂, N₂ and CH₄ have strong covalent bonds inside the molecule but weak forces between molecules, so they have low melting points and do not conduct.
- 4
Giant covalent structures (diamond, graphite, silicon dioxide) have a network of strong covalent bonds, so they have very high melting points.
- 5
Diamond: each carbon bonded to four others, tetrahedral, hard, does not conduct. Graphite: each carbon bonded to three others in layers, delocalised electrons, so it conducts and the layers can slide (lubricant, pencils).
- 6
Metals consist of positive ions in a sea of delocalised electrons. Metallic bonding explains conductivity, malleability and high melting points of many metals.
- 7
The properties of a substance are explained by its bonding and structure. Learn a named example for each type and the reason, not just the label.
- 8
Graphene is a single layer of graphite. Fullerenes are cages or tubes of carbon. They have large surface areas and, in some cases, delocalised electrons, which is why they appear in materials questions.
Go deeper
Bonding names the force; structure names the pattern
Students write “ionic” when the mark scheme wanted “giant ionic lattice”, or “covalent” when it wanted “simple molecules”. Bonding is the attraction. Structure is how the particles are arranged. Sodium chloride is ionic bonding in a giant lattice: strong electrostatic forces, high melting point, conducts when ions are free to move. Water is covalent bonding in simple molecules: the O–H bonds are strong, but the intermolecular forces are weak, so water boils at 100 °C, not at diamond temperatures. If you only say “strong bonds”, an examiner cannot tell whether you mean inside the molecule or between molecules. That distinction is the difference between a grade 4 and a grade 7 answer.
Go deeper
Carbon allotropes are a three-way comparison
Diamond, graphite and graphene are all carbon. The difference is how many covalent bonds each atom makes and whether electrons are free. In diamond, four bonds per carbon, no delocalised electrons: hard, high melting point, insulator, used in cutting tools. In graphite, three bonds, layers, one delocalised electron per carbon: conducts electricity, layers slide, used in electrodes and as a lubricant. Graphene is one hexagon layer: very strong for its thickness and conducts. Silicon dioxide is another giant covalent structure, used to make you compare oxides with carbon. Draw a small sketch in revision: tetrahedron versus hexagonal layers. Then attach one property and one use to each sketch.
Go deeper
Metals and molten salts both conduct — for different reasons
Electrical conduction needs charged particles that can move. In metals those particles are delocalised electrons, which is why a copper wire conducts while solid. In ionic compounds the particles are ions, which are locked in the lattice until the solid melts or dissolves. A common trap is “metals conduct because they have ions” or “salt conducts because of electrons”. Match the mobile charge carrier to the structure. Malleability is a metallic story: layers of ions can slide over each other and the delocalised electrons hold the metal together. Ionic crystals shatter because like charges become aligned when the layers shift. That comparison is a favourite six-mark.
See the idea in action
Explain why graphite conducts electricity but diamond does not. Both are giant covalent carbon. In graphite each carbon forms three covalent bonds, so one electron per carbon is delocalised and can carry charge through the layers. In diamond each carbon forms four covalent bonds, so there are no delocalised electrons and charge cannot flow. Both have high melting points because strong covalent bonds must be broken to melt the giant structure. Graphite is soft because layers are held by weak forces and can slide.
Exam technique
Turn knowledge into marks
Always name the bonding, the structure, the force that must be overcome, and the mobile particle if conductivity is asked. “It has strong bonds” is not enough. For simple molecules, say that covalent bonds stay intact and intermolecular forces are overcome on boiling.
Common mistakes
Do not give these marks away
- 01
Saying ionic compounds conduct as solids, or that covalent molecules have weak covalent bonds.
- 02
Explaining graphite’s conductivity with “layers slide” — sliding explains softness, delocalised electrons explain conduction.
- 03
Claiming metals are ionic, or that diamond conducts because it is carbon.
Why does graphite conduct electricity when diamond does not?
AGraphite has ionic bonds
BGraphite has delocalised electrons; diamond does not
CDiamond is a metal
DGraphite molecules are small and can move
Show the answer
Graphite has delocalised electrons; diamond does not. Each carbon in graphite bonds to three others, leaving delocalised electrons that carry charge. In diamond every outer electron is in a covalent bond.
Quick questions
If this is the bit you searched
Why do ionic compounds have high melting points?
They form giant lattices with strong electrostatic forces between oppositely charged ions acting in all directions. A lot of energy is needed to break the lattice.
Why do simple covalent molecules have low boiling points?
The covalent bonds inside each molecule are strong, but the forces between molecules are weak, so little energy is needed to separate the molecules.
Why do metals conduct electricity?
Delocalised electrons can move through the metallic structure and carry charge.