Question

Which of the following metals does not forms solid hydrogen carbonates?

• A. Li and Na
• B. Na and K
• C. Li and Mg
• D. Li and K

Answer 1

The correct option is C Li and Mg

All alkali metals forms solid hydrogen carbonates except Li due to its exceptionally smaller size and high polarising power.
Li and Mg have diagonal relationship due to their similar size and they have some similar properties. Thus, both Li and Mg does not form solid hydrogen carbonates. Hence, option (c) is correct.

Theory
Similarities between Li and Mg
Lithium shows a diagonal relationship with magnesium.

Similarity arises because of their similar size
Li - 152 pm and $L{i}^{+}$ - 76 pm
Mg - 160 pm and $M{g}^{2+}$ - 72 pm
1. Both Li and Mg are harder and lighter.
2. Li and Mg react slowly with water
3. $L{i}_{2}O$ and MgO do not combine with excess oxygen to give any superoxide
4. Their carbonates decompose easily on heating to form the oxides and $C{O}_2$

Note:- Solid hydrogen carbonates are not formed by Li and Mg
5. Both LiCl and $MgC{l}_2$ are soluble in ethanol, i.e., chlorides of Li and Mg are organic.
6. Both LiCl and $MgC{l}_2$ are deliquescent and crystallize from aqueous solutions
as hydrates, $LiCl.2H_{2}O$ and $MgC{l}_2.8H_{2}O$