An alkaline earth metal ‘M’ readily forms water soluble sulphate and water insoluble hydroxide. Its oxide MO is very stable to heat and does not have rock-salt structure. M is :

Solution

Chemistry ma’am · 3 steps

The first two clues still leave Mg and Be; only the oxide's crystal structure picks Be.

1. Sulphate and hydroxide. Down group 2, sulphates become less soluble and hydroxides become more soluble. A soluble sulphate with an insoluble hydroxide points to the top of the group, so the choice is Be or Mg.

   Metal	MSO₄ in water	M(OH)₂ in water	Structure of MO
   Be	soluble	insoluble	wurtzite (4:4), not rock salt
   Mg	soluble	insoluble	rock salt (6:6)
   Ca	sparingly soluble	sparingly soluble	rock salt (6:6)
   Sr	insoluble	fairly soluble	rock salt (6:6)

2. Why BeO differs. Be²⁺ is so small that only four O²⁻ ions fit around it, so BeO takes the wurtzite structure. MgO, CaO and SrO all have the six-coordinate rock-salt structure. BeO is also very stable to heat, with a melting point above 2500 °C.
3. So. Only beryllium fits every clue, so M is Be, option (d).

The idea to keep. Group 2 trends: sulphate solubility falls and hydroxide solubility rises down the group. When Be and Mg both fit, look for Be's small-ion anomaly, like the wurtzite structure of BeO.

Answer from the NTA final answer key. Solution by PrepWiser.