Q.Alkaline earth metal (A), belongs to 3rd period reacts with oxygen and nitrogen to form compound (B) and (C) respectively. It undergoes metal displacement reaction with AgNO₃ solution to form compound (D). Identify A,B.C and D.

Step 1. 'Alkaline earth metal (A), belongs to the 3rd period': among Be, Mg, Ca, Sr, Ba, Ra, the one in period 3 is magnesium (Mg, atomic number 12) -- so A = Mg.

Step 2. 'Reacts with oxygen to form compound (B)': magnesium burns in oxygen to give the normal oxide, 2Mg + O₂ → 2MgO -- so B = MgO.

Step 3. 'Reacts with nitrogen to form compound (C)': magnesium, like lithium, combines directly with nitrogen to give the nitride, 3Mg + N₂ → Mg₃N₂ -- so C = Mg₃N₂. …

Use each clue in turn (period-3 alkaline earth metal; product with O2; product with N2; displacement product with AgNO3) to pin down A, …

• Identifying A as calcium instead of magnesium -- calcium is a period-4 element (K, Ca are period 4), so the 'belongs to the 3rd period' clue specifically singles out magnesium. …