Two elements A and B have similar chemical properties. They don’t form solid hydrogencarbonates, but react with nitrogen to form nitrides. A and B, respectively, are :

Solution

Chemistry ma’am · 3 steps

Lithium is the odd one out among the alkali metals, and it copies its diagonal partner, magnesium.

1. No solid bicarbonate. NaHCO₃, KHCO₃, RbHCO₃ and CsHCO₃ all exist as solids. LiHCO₃ does not, and neither do the hydrogencarbonates of Group 2 metals such as Mg(HCO₃)₂ and Ca(HCO₃)₂. These are found only in solution.
2. Forms a nitride. Lithium is the only alkali metal that reacts with nitrogen: 6Li + N₂ → 2Li₃N. Group 2 metals do the same when heated, for example 3Mg + N₂ → Mg₃N₂.

   Option	Element A	Element B	Both pass?
   (a)	Li: no solid LiHCO₃, forms Li₃N	Mg: no solid Mg(HCO₃)₂, forms Mg₃N₂	yes
   (b)	Cs: solid CsHCO₃, no nitride	Ba: passes	no, Cs fails
   (c)	Na: solid NaHCO₃, no nitride	Rb: solid RbHCO₃, no nitride	no, both fail
   (d)	Na: solid NaHCO₃, no nitride	Ca: passes	no, Na fails

3. So. Only Li and Mg pass both tests. Their similar chemistry comes from the diagonal relationship: both have small atoms and high charge density. The answer is (a).

The idea to keep. Whenever lithium behaves unlike the other alkali metals (nitride formation, no solid bicarbonate, covalent character), it is behaving like magnesium. That is the diagonal relationship.

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