Structure and Properties of Substances

The four types of structure

1. Giant ionic lattice

Formed by ionic compounds (e.g. NaCl, MgO). Millions of ions in a regular 3-D lattice held by strong electrostatic forces in all directions.

Properties:

• High melting and boiling points (strong ionic bonds throughout the lattice require large energy to break).
• Conduct electricity only when molten or dissolved in water (ions must be free to move; in solid state, ions are fixed).
• Soluble in polar solvents (e.g. water) — water molecules surround and pull apart ions.
• Hard but brittle (a displaced layer brings like charges together → repulsion → shatters).

2. Simple molecular (covalent)

Small discrete molecules with strong covalent bonds within the molecule but only weak intermolecular forces (van der Waals / London dispersion forces) between molecules (e.g. H₂O, CO₂, NH₃, Cl₂, H₂, CH₄).

Properties:

• Low melting and boiling points (little energy to overcome the weak intermolecular forces).
• Do NOT conduct electricity (no charged particles free to move; electrons are localised within bonds).
• Volatility — many are gases or liquids at room temperature.

Note: water has a relatively high boiling point (100 °C) for its small molecular size due to hydrogen bonding — but this is an A-level detail; at GCSE state "stronger intermolecular forces than similar-sized molecules."

3. Giant covalent (macromolecular) structures

Each atom is bonded to several others by strong covalent bonds throughout the whole structure, forming a giant 3-D network. Examples: diamond, graphite, silicon dioxide (SiO₂).

Diamond:

• Each C is covalently bonded to 4 others in a tetrahedral arrangement.
• All electrons are in localised bonds → does NOT conduct electricity.
• Very hard (must break covalent bonds to deform) — used in cutting tools.
• Very high melting point.

Graphite:

• Each C is covalently bonded to 3 others in hexagonal layers.
• Each C has 1 delocalised electron → conducts electricity (used as electrodes).
• Weak forces between layers → layers slide → soft/lubricant.
• High melting point (strong covalent bonds within layers).

Silicon dioxide (SiO₂):

• Giant covalent network; each Si bonded to 4 O atoms.
• High melting point; does not conduct electricity; insoluble in water.

4. Metallic structure

(Covered in bonding section.) Lattice of positive ions + sea of delocalised electrons → high melting point, good conductor, malleable/ductile.

Summary comparison table

CCEA exam tip

"Giant" structures have high melting points — the word "giant" tells you there are many strong bonds throughout to break. "Simple molecular" substances have low melting points — only weak intermolecular forces break during melting, not the covalent bonds themselves.