Thermal stability of metal carbonates increases with decreasing cation charge density

Steps:

• Identify cations: Ca²⁺ (Group 2), Cu²⁺/Zn²⁺ (transition metals), Pb²⁺ (post-transition).
• Recall that stability of MCO₃ decreases with higher charge density of M²⁺, as it polarizes CO₃²⁻ more easily, favoring decomposition to MO + CO₂.
• Compare sizes: Ca²⁺ (100 pm) has lower charge density than Cu²⁺ (73 pm) or Zn²⁺ (74 pm); Pb²⁺ (119 pm) still decomposes easier due to oxide stability.
• Conclude CaCO₃ requires highest temperature (~840°C) for decomposition.

Why A is correct:

• Group 2 carbonates follow the trend where stability increases down the group due to larger cation size reducing polarization of CO₃²⁻ (Fajans' rules).

Why the others are wrong:

• B: Cu²⁺ has high charge density, causing decomposition at ~200°C.
• C: PbCO₃ decomposes at ~315°C, as PbO is stable.
• D: Zn²⁺ high charge density leads to decomposition at ~300°C.

Final answer: A