Ionic vs covalent bonding (and metallic bonding)

In sodium chloride, sodium (2,8,1) loses its outer electron to become Na⁺ (2,8), and chlorine (2,8,7) gains it to become Cl⁻ (2,8,8). The oppositely charged ions attract strongly in all directions, forming a giant ionic lattice.

In magnesium oxide, magnesium loses two electrons (Mg²⁺) and oxygen gains two (O²⁻). The higher charges make the attraction stronger, so MgO has a much higher melting point than NaCl.

Non-metal atoms share pairs of electrons. Each shared pair is one covalent bond:

• H₂O: oxygen shares one pair with each hydrogen (two single bonds).
• CO₂: carbon shares two pairs with each oxygen (two double bonds).
• CH₄: carbon shares one pair with each of four hydrogens.

In dot-and-cross diagrams, show only the outer shells and use dots for one atom's electrons and crosses for the other's.

Graphite conducts electricity because each carbon forms three bonds, leaving one delocalised electron per atom free to move. It's soft because its layers can slide over each other.

Metals are a lattice of positive ions surrounded by delocalised electrons. The electrons can move, so metals conduct electricity and heat. The layers of ions can slide, so metals are malleable. Alloys are harder because different-sized atoms disrupt the layers so they can't slide as easily.

Ionic compounds conduct when molten or dissolved because the ions are then free to move.

• Saying ionic compounds conduct because electrons move: it's the ions that move.
• Saying covalent bonds break when a simple molecular substance melts: it's the weak forces between molecules.
• Forgetting the charges in ionic dot-and-cross diagrams.

1. What type of bonding is in sodium chloride?
2. What type of bonding is in carbon dioxide?
3. What type of bonding is in copper?
4. Diamond and methane are both covalent. Why does diamond have a much higher melting point?

Answers: ionic; covalent (simple molecules); metallic; diamond is a giant covalent structure, so melting it breaks many strong covalent bonds, while melting methane only overcomes weak forces between molecules.