Group 2 trends: carbonate stability and ionisation energy

Steps:

• Carbonate decomposition temperature rises down Group 2 as cation size increases, stabilizing the lattice and requiring higher heat to decompose MCO₃ → MO + CO₂.
• First ionisation energy falls down Group 2 due to larger atomic radius and increased shielding, reducing attraction between nucleus and outer electron.
• From Mg to Ba, MgCO₃ decomposes at lower temperature (~540°C) than BaCO₃ (~1360°C), confirming increase.
• Ionisation energies decrease: Mg (738 kJ/mol) > Ba (503 kJ/mol), confirming decrease.

Why D is correct:

• Matches observed trends: larger ions enhance carbonate thermal stability (lattice energy rule), while atomic size increase lowers ionisation energy (effective nuclear charge decreases).

Why the others are wrong:

• A: Ionisation energy decreases, not decreases.
• B: Carbonate decomposition temperature increases, not decreases.
• C: Both trends are opposite—increases for decomposition, decreases for ionisation.

Final answer: D