Ionic lattices and covalent bonds
A giant lattice of ions
In a solid ionic compound, huge numbers of positive and negative ions sit in a regular three-dimensional pattern called a giant ionic lattice.
Every ion is attracted to the oppositely charged ions around it by strong electrostatic forces acting in all directions.
That explains the properties:
- High melting and boiling points: melting means overcoming a vast number of strong attractions, which takes a lot of energy.
- No conduction as a solid: the ions are locked in place.
- Conducts when molten or dissolved in water: the ions are free to move and carry charge.
It is the ions that move, not electrons. Writing "electrons move" in an ionic answer loses the mark.
Sharing instead of transferring
Two non-metal atoms both need electrons, so neither gives any away.
They share a pair of electrons: one from each atom. A shared pair is a covalent bond, and it is strong.
In water, oxygen (6 outer electrons) shares one pair with each hydrogen atom. Oxygen now has 8 in its outer shell and each hydrogen has 2. The two pairs on oxygen that are not shared are non-bonding pairs.
Counting shared pairs
Each atom shares as many electrons as it needs to fill its outer shell (hydrogen needs 2, the others 8).
- H₂, Cl₂, HCl: 1 shared pair each
- H₂O: 2 shared pairs (O needs 2 more)
- NH₃: 3 shared pairs (N needs 3 more)
- CH₄: 4 shared pairs (C needs 4 more)
- O₂: a double bond, 2 shared pairs
- N₂: a triple bond, 3 shared pairs
Electrons in bonds = 2 × the number of shared pairs.
Metal with non-metal: ionic. Non-metals only: covalent. A metal on its own: metallic (lesson 5).
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