Thermal decomposition of Group 2 nitrates yields two gases with stability increasing down the group

Steps:

• Decomposition producing two gases identifies nitrates, as 2M(NO3)2 → 2MO + 4NO2 + O2.
• Carbonates decompose to one gas: MO + CO2.
• Low thermal stability compared to other Group 2 compounds with same anion points to magnesium nitrate, as stability rises down Group 2 due to decreasing charge density.
• Barium compounds are more stable, confirming magnesium nitrate as Z.

Why D is correct:

• Mg(NO3)2 decomposes to MgO + NO2 + O2 (two gases); its low stability follows Fajans' rules, where small Mg2+ highly polarizes and destabilizes NO3- relative to larger cations.

Why the others are wrong:

• A. Barium carbonate decomposes to BaO + CO2 (one gas).
• B. Barium nitrate has higher thermal stability than magnesium nitrate.
• C. Magnesium carbonate decomposes to MgO + CO2 (one gas).

Final answer: D