Group 2 Trends: Carbonate Stability and Ionisation Energy

Steps:

• Atomic radius increases down Group 2 from Mg to Ba, shielding valence electrons from the nucleus.
• First ionisation energy decreases as removing an outer electron becomes easier with greater distance and shielding.
• Carbonate ion (CO3^2-) is large and polarizable; smaller Mg^2+ distorts it more, leading to lower decomposition temperature.
• Larger Ba^2+ causes less distortion, increasing thermal stability and thus decomposition temperature.

Why D is correct:

• Down Group 2, increasing cation size reduces polarizing power on CO3^2- (Fajans' rules), raising decomposition temperature, while larger atomic radius lowers first ionisation energy.

Why the others are wrong:

• A: Carbonate decomposition temperature increases, not decreases.
• B: Both trends are reversed; decomposition temperature increases and ionisation energy decreases.
• C: First ionisation energy decreases, not increases.

Final answer: D