The ionic model practice questions.

State the formula of the ionic compound formed between aluminium and oxygen.

• AlO
• Al2O3
• Al3O2
• AlO2

Answer: Al2O3

Al forms Al³⁺ and O forms O²⁻; balancing charges gives Al2O3 (2×3 = 3×2 = 6).

Determine the formula of the compound formed between calcium ions and phosphate ions.

• CaPO4
• Ca2(PO4)3
• Ca3(PO4)2
• Ca(PO4)2

Answer: Ca3(PO4)2

Ca²⁺ with PO₄³⁻: to balance charge you need three Ca²⁺ (+6) and two PO₄³⁻ (−6), giving Ca3(PO4)2.

Which factors together give the strongest ionic bonding?

• Large ionic radii and low ionic charges
• Small ionic radii and high ionic charges
• Large ionic radii and high ionic charges
• Small ionic radii and low ionic charges

Answer: Small ionic radii and high ionic charges

Ionic bond strength increases with higher ionic charge and smaller ionic radius, since the electrostatic attraction is proportional to (q⁺q⁻)/r.

Which of these ionic compounds is expected to have the highest melting point?

• NaCl
• KCl
• MgO
• CaO

Answer: MgO

MgO has doubly charged ions (Mg²⁺, O²⁻) and small ionic radii, giving the strongest electrostatic attraction and therefore the highest melting point.

What is the charge on the ion typically formed by an element in group 16 (group VI) of the periodic table?

• 1+
• 2+
• 2−
• 6−

Answer: 2−

Group 16 elements gain two electrons to reach a noble-gas configuration, forming a 2− ion (e.g. O²⁻, S²⁻).

Which statement best explains why molten sodium chloride conducts electricity but solid sodium chloride does not?

• Delocalised electrons are free to move only when the NaCl is molten
• In the molten state ions are free to move and carry charge
• Solid NaCl contains no ions
• Melting converts the ionic bonds into covalent bonds

Answer: In the molten state ions are free to move and carry charge

Conduction requires mobile charge carriers; in the solid, ions are fixed in the lattice, but on melting the ions become free to move.