Ionic and covalent compounds: how bonding sets properties

Ions in a lattice, or separate molecules

In ionic compounds, oppositely charged ions are held in a crystal lattice by strong electrostatic forces. Breaking it takes a lot of energy, so melting points are high: sodium chloride melts at 801 °C.

Ionic solids are brittle and do not conduct, because the ions cannot move. Molten or dissolved in water, the ions are free, so they conduct. Water also hydrates the ions, which is why many ionic solids dissolve.

Covalent compounds are mostly separate neutral molecules made of non-metals. The forces between molecules are weak, so small ones are gases or low-boiling liquids. They are usually bad conductors and usually insoluble in water.

Carbon shows how structure decides use. In diamond each carbon is bonded to four others, so it is very hard: cutting, polishing and drilling tools. Graphite's layers slip and carry mobile electrons: a lubricant and an electrode.

Conduction needs free charged particles: ions that can move, or mobile electrons.

Key terms

Brittle

Describing ionic solids under force.

Hydration

Explaining why ionic solids dissolve in water.

Electrolysis

Making elements from molten or dissolved ionic compounds.

Lubricant

Graphite's use, because its layers slip.

Short questions with model answers

1. Q1. Solid NaCl does not conduct, but molten NaCl does. Why? (p. 46)

   • Solid: ions fixed in the lattice
   • Molten: ions free to move

   Conduction needs moving charges; melting frees the ions.

2. Q2. What happens when NaBr is treated with AgNO₃ in water? (CRQ v, p. 51; compare p. 47)

   • NaBr(aq) → Na⁺ + Br⁻; AgNO₃(aq) → Ag⁺ + NO₃⁻
   • Ag⁺ + Br⁻ → AgBr(s)

   A precipitate of silver bromide forms; Na⁺ and NO₃⁻ stay in solution.

Common mistakes

• ✗ Solid salt conducts electricity because it is made of ions.

  ✓ In the solid, the ions are held tightly and cannot move, so it does not conduct.

• ✗ Covalent compounds boil easily because their covalent bonds are weak.

  ✓ The bonds inside molecules are strong; the forces between molecules are weak.

• ✗ Diamond and graphite are both carbon, so they have the same uses.

  ✓ Their structures differ: rigid diamond cuts, layered graphite lubricates and conducts.

MCQs

1. 1. Which form of carbon is used as a lubricant?

   1. (a) Coal
   2. (b) Diamond
   3. (c) Graphite
   4. (d) Charcoal
   Show answer

   (c) MCQ (v), p. 50; p. 48: graphite's layers slip past each other.

2. 2. Ionic compounds conduct electricity:

   1. (a) Only as solids
   2. (b) Never
   3. (c) Only as gases
   4. (d) When molten or dissolved in water
   Show answer

   (d) p. 44 table and p. 46: free ions are present only when molten or in solution.

3. 3. Why is diamond used in cutting and drilling tools?

   1. (a) Each carbon is strongly bonded to four others
   2. (b) Its layers slip
   3. (c) It conducts electricity
   4. (d) It is a metal
   Show answer

   (a) p. 48: the rigid structure of strong covalent bonds makes diamond very hard.

4. 4. Most covalent compounds with low molecular mass are:

   1. (a) High-melting crystals
   2. (b) Gases or low-boiling liquids
   3. (c) Metals
   4. (d) Good conductors
   Show answer

   (b) p. 44: low molecular mass covalent compounds are gases or low-boiling liquids, e.g. CH₄, NH₃, CO₂ (p. 48).

5. 5. Why does sodium chloride have a high melting point (801 °C)?

   1. (a) Its molecules are heavy
   2. (b) It contains hydrogen bonds
   3. (c) Strong electrostatic forces between ions are hard to break
   4. (d) It is covalent
   Show answer

   (c) p. 44: it is difficult to break the strong electrostatic attraction between the ions.

Quick revision

• Ionic compounds: high melting points, brittle, soluble in water, conduct when molten or dissolved.
• Covalent compounds: separate molecules, often gases or low-boiling liquids, usually non-conductors.
• Structure sets use: diamond cuts; graphite lubricates and works as an electrode.