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.

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.

Each atom shares as many electrons as it needs to fill its outer shell (hydrogen needs 2, the others 8).

1. H₂, Cl₂, HCl: 1 shared pair each
2. H₂O: 2 shared pairs (O needs 2 more)
3. NH₃: 3 shared pairs (N needs 3 more)
4. CH₄: 4 shared pairs (C needs 4 more)
5. O₂: a double bond, 2 shared pairs
6. 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).