Diagonal relationship between lithium and magnesium — explain with examples

In the periodic table, elements show a diagonal relationship when an element in one group resembles the element diagonally placed in the next period and next group more closely than it resembles the elements directly below it in its own group.

The three classic diagonal pairs:

• Li (Group 1, Period 2) ↔ Mg (Group 2, Period 3)
• Be (Group 2, Period 2) ↔ Al (Group 13, Period 3)
• B (Group 13, Period 2) ↔ Si (Group 14, Period 3)

The diagonal relationship arises from a similarity in ionic charge density (charge/size ratio) of the diagonally placed elements. As we move across a period, atomic size decreases (nuclear charge increases). As we move down a group, atomic size increases. These two trends partially cancel each other along a diagonal, resulting in similar charge densities.

Li$^+$ radius ≈ 76 pm. Mg$^{2+}$ radius ≈ 72 pm. Both have high charge-to-radius ratios (charge density) — Li$^+$ with charge 1 and Mg$^{2+}$ with charge 2 but larger radius effectively have comparable polarising power.

1. Behaviour with oxygen: Both Li and Mg form normal oxides when burned in air. Na (just below Li in Group 1) forms sodium peroxide ($Na_2O_2$), but:

• Li burns in $O_2$ to form $Li_2O$ (normal oxide)
• Mg burns in $O_2$ to form $MgO$ (normal oxide)

2. Nitride formation: Both react directly with nitrogen to form nitrides. Na and K do NOT do this readily.

• $6Li + N_2 \rightarrow 2Li_3N$ (lithium nitride)
• $3Mg + N_2 \rightarrow Mg_3N_2$ (magnesium nitride)

3. Carbonate stability: Both $Li_2CO_3$ and $MgCO_3$ decompose on heating to form oxides. Higher alkali metal carbonates ($Na_2CO_3$, $K_2CO_3$) are stable to heat.

4. Bicarbonate: $LiHCO_3$ is unstable and exists only in solution. Same for $Mg(HCO_3)_2$. Higher alkali metal bicarbonates are solid.

5. Solubility of fluorides and hydroxides: $LiF$ and $MgF_2$ are sparingly soluble, unlike $NaF$ (very soluble). $LiOH$ and $Mg(OH)_2$ are sparingly soluble; $NaOH$ is very soluble.

6. Organometallic compounds: Both form organometallic compounds readily. Grignard reagents ($RMgX$) are analogous to organolithium compounds ($RLi$). Higher alkali metals (Na, K) don’t form stable organometallics of the same type.