Group 2: Question 7

Worked solution

Part (a): Magnesium burning in oxygen

Magnesium is a Group 2 metal and forms a $2+$ ion, so it combines with oxygen in a $2:1:2$ ratio to give the simple oxide:

$2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$

The formula $\text{MgO}$ (not $\text{Mg}_2\text{O}$ or $\text{MgO}_2$) follows directly from the $2+$ charge on $\text{Mg}^{2+}$ balancing the $2-$ charge on $\text{O}^{2-}$.

Observation: the magnesium burns with a brilliant, blinding white flame/light, leaving behind a white solid (magnesium oxide “ash”).

Part (b): Calcium burning in oxygen

Calcium behaves in the same general way as magnesium, also forming a $2+$ ion and combining with oxygen in the same ratio:

$2\text{Ca(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CaO(s)}$

Part (c): Why calcium reacts more vigorously than magnesium

Going down Group 2, atomic radius increases and first ionisation energy decreases, because the outer $ns^2$ electrons are further from the nucleus and are shielded by an extra inner shell of electrons. Calcium, being below magnesium, therefore has a larger atomic radius and a lower first ionisation energy than magnesium. Its outer two electrons are more easily lost to oxygen atoms, so calcium reacts faster and more vigorously with oxygen than magnesium does.

Final answers

• (a) $2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$; brilliant white flame, leaving a white solid.
• (b) $2\text{Ca(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CaO(s)}$
• (c) Calcium’s larger atomic radius and lower first ionisation energy (than magnesium’s) make its outer electrons easier to lose, so it reacts more vigorously with oxygen.