Identify the structural formula for magnesium hydroxide.

Correct — A. Magnesium is a Group 2 metal and forms Mg²⁺. Hydroxide is OH⁻.

One +2 charge needs two −1 charges to balance, so two hydroxide groups are required, and a repeated group is written inside a bracket: Mg(OH)₂.

Option (a) is that bracketed form — the response sheet has lost the subscript in printing, so it appears as Mg(OH). No other option uses the bracket the formula demands.

Why the others are wrong

• (b)
  MgO — MgO is magnesium oxide, the white ash left when magnesium burns in air. It contains oxygen but no hydroxide group at all.
• (c)
  Mg — Mg is the element itself, magnesium metal. A bare symbol names a substance, not a compound of magnesium with hydroxide.
• (d)
  MgOH — MgOH pairs one Mg²⁺ with a single OH⁻, leaving a net charge of +1. A neutral compound must balance, so a second hydroxide is needed.

Write an ionic formula by balancing charges. Mg²⁺ needs two OH⁻ ions, giving Mg(OH)₂.

The bracket is not decoration. It tells you the subscript applies to the whole hydroxide group, so the compound holds one magnesium, two oxygens and two hydrogens.

Magnesium hydroxide is a sparingly soluble base. In suspension it is milk of magnesia, used as an antacid and a mild laxative.

Key facts

• Magnesium hydroxide is Mg(OH)₂.
• Magnesium forms the Mg²⁺ ion, so two OH⁻ ions are needed for a neutral compound.
• The bracket in Mg(OH)₂ shows the subscript applies to the whole hydroxide group.
• Magnesium hydroxide in suspension is milk of magnesia, used as an antacid.

Common traps

• Choosing MgOH because it looks like a hydroxide, without checking that the charges balance.
• Confusing magnesium oxide, MgO, with magnesium hydroxide.

No directly related past PYQ was found.