Q.In what ways lithium shows similarities to magnesium in its chemical behaviour?

Step 1: Why the resemblance exists

Li (Group 1) and Mg (Group 2) are diagonal neighbours in the periodic table. Their ionic radii and charge/size (polarising power) are similar, so Li$^+$ behaves chemically much more like Mg$^{2+}$ than like the other alkali metal ions — this is the classic diagonal relationship.

Step 2: Points of resemblance

• Both Li and Mg are harder and have higher melting points than the other members of their respective groups.
• Both react directly with N$_2$ to form a nitride ($6Li + N_2 \rightarrow 2Li_3N$; $3Mg + N_2 \rightarrow Mg_3N_2$) — no other alkali metal forms a nitride this readily.
• LiOH and Mg(OH)$_2$ are both comparatively weak bases and decompose on heating to the oxide.
• Li$_2$CO$_3$ and MgCO$_3$ are both thermally unstable, decomposing on heating to the oxide + CO$_2$ (unlike Na$_2$CO$_3$, which is stable to strong heating). …

Invoke the diagonal relationship: Li$^+$ and Mg$^{2+}$ have comparable charge density, so Li mimics Mg rather than Na in nitride formation, thermal instability of carbona …

• Comparing Li to Ca or Ba instead of Mg — the diagonal relationship is specifically Li–Mg (and Be–Al).
• Forgetting to explain why the resemblance exists (similar charge/radius ratio) and only listing similarities. …