State the meaning of the formula MgCl₂ and describe what information the subscript '2' provides about the compound.

Cambridge International IGCSE Chemistry (0620) — 3.1 Formulae · State · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Magnesium chloride has the formula MgCl₂.

Model answer (4 marks)

The formula MgCl₂ shows that the compound contains magnesium and chlorine atoms. It indicates the ratio of atoms in the compound, which is 1 magnesium to 2 chlorine atoms. The subscript ‘2’ tells us that there are two chlorine atoms for every one magnesium atom. MgCl₂ is the simplest or empirical formula, giving the simplest whole‑number ratio of atoms in the compound.

Examiner tips

Use the word ‘contains’ to state the elements. Mention the 1:2 ratio. Explain the subscript as the number of atoms. State that it is the simplest whole‑number ratio (empirical formula).

Common mistakes

Writing the ratio as 2:1 instead of 1:2. Forgetting to mention that the formula is the simplest whole‑number ratio. Using vague terms like ‘shows’ without specifying the elements or ratio.

Mark scheme (4 marks)

The formula shows magnesium chloride contains magnesium and chlorine (atoms/elements)
The formula shows the ratio of atoms in the compound
The subscript '2' means there are two chlorine atoms for every one magnesium atom
MgCl₂ is the simplest / empirical formula showing the simplest whole-number ratio of atoms in the compound

Key terms in this question

formula · subscript