Bonding, Structure and Properties (C2.2)

States of matter and particle model

• Solid: particles closely packed in fixed positions; vibrate in place; fixed shape and volume.
• Liquid: particles close together but can flow past each other; fixed volume, no fixed shape.
• Gas: particles far apart and moving rapidly in all directions; no fixed shape or volume; easily compressed.

Changes of state: require overcoming intermolecular forces (not breaking covalent bonds in simple molecules). Melting, boiling, evaporation, condensation, freezing, sublimation.

Ionic compounds (e.g. NaCl, MgO)

• Giant ionic lattice — millions of ions in a regular 3D arrangement.
• High melting and boiling points — many strong electrostatic forces to overcome.
• Conduct electricity when molten or in aqueous solution — ions free to move.
• Do not conduct when solid — ions fixed in lattice.
• Soluble in water (many ionic compounds) — water molecules surround and pull apart ions.

Simple molecular substances (e.g. H₂O, CO₂, CH₄)

• Small discrete molecules; covalent bonds within molecules are strong.
• Intermolecular forces between molecules are weak.
• Low melting/boiling points — little energy to overcome weak intermolecular forces.
• Do not conduct electricity — no charged particles free to move.
• May be soluble or insoluble depending on polarity.

Polymers (e.g. poly(ethene))

• Long chains of repeating monomer units.
• Molecules are larger than simple molecular substances → intermolecular forces stronger → higher melting point than small molecules.
• Still relatively low melting point compared to giant structures.
• Do not conduct electricity.

Giant covalent structures (e.g. diamond, graphite, SiO₂)

• Atoms joined by strong covalent bonds extending throughout the whole structure.
• Very high melting/boiling points — many strong covalent bonds to break.
• Diamond: each carbon bonded to 4 others in tetrahedral arrangement → very hard; does not conduct.
• Graphite: each carbon bonded to 3 others in hexagonal layers; one delocalised electron per carbon → conducts electricity; layers can slide → lubricant; used in pencils.
• Graphene: single layer of graphite; exceptional conductor; used in electronics.

Metallic structures

• Giant lattice of positive ions in a sea of delocalised electrons.
• Good conductors (heat and electricity), high melting points (generally), malleable and ductile.

Common exam errors

1. Saying covalent bonds break when a simple molecular compound melts — no, intermolecular forces break.
2. Forgetting graphite conducts because of delocalised electrons (not because of the layers).
3. Saying ionic compounds conduct in all states — only molten or in solution, NOT solid.