2018 O Level Chem P1 Q7 — Chemical Bonding / Ionic Formulae Solution
Question
A compound contains two elements, metal Y and non-metal Z. The compound consists of a lattice of positive ions and negative ions. Each positive ion is surrounded by eight negative ions and each negative ion is surrounded by four positive ions. Which ions are present in the compound, and what is its formula?
Correct answer: B
Video Solution
Explanation
Answer: B (\(\text{Y}^{2+}\) and \(\text{Z}^{-}\), formula \(\text{YZ}_2\)). The ratio of positive to negative ions is found by inverting the coordination numbers. Each positive ion is surrounded by 8 negative ions and each negative ion by 4 positive ions, so positive : negative = 4 : 8 = 1 : 2.
The formula unit therefore has one positive ion to two negative ions: \(\text{YZ}_2\). For the compound to be neutral, \((\text{charge on Y}) + 2 \times (\text{charge on Z}) = 0\), so Y must be \(2+\) and Z must be \(1-\).
Only option B gives \(\text{Y}^{2+}\), \(\text{Z}^{-}\) and \(\text{YZ}_2\). Answer B.
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