ZIMSEC O Level · Chemistry · Form 3
Atoms bond to get a full outer shell.
Ionic bonding
Between metals and non-metals. Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions. The ions are held by strong electrostatic attraction in a giant lattice.
Properties: high melting points; conduct electricity when molten or dissolved (ions free to move), not as solids; often soluble in water. Example: NaCl.
Covalent bonding
Between non-metals, which share pairs of electrons.
- Simple molecules (H₂O, CO₂, CH₄, Cl₂): weak forces between molecules, so low melting and boiling points; do not conduct.
- Giant covalent structures: diamond (each C bonded to 4 others – very hard, used in cutting tools) and graphite (layers that slide – soft, lubricant, conducts because of free electrons).
Metallic bonding
A lattice of positive ions in a “sea” of delocalised electrons. Metals conduct heat and electricity and are malleable.
Worked example: Draw the dot-and-cross diagram for water.
Oxygen (6 outer electrons) shares one pair with each of two hydrogen atoms, giving O two bonding pairs and two lone pairs, and each H a full shell of 2.
Check yourself
- Why does sodium chloride conduct when molten?
- Explain why graphite can be used as a lubricant.
- Why does methane have a low boiling point?
Show answers
- The ions become free to move and carry charge.
- Its layers are held by weak forces and slide over each other.
- The forces between the molecules are weak, so little energy is needed to separate them.
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