Explain diagonal relationship between Li-Mg and Be-Al with examples

Question

What is diagonal relationship in the periodic table? Explain the similarities between Li-Mg and Be-Al with at least three examples for each pair.

(NCERT Class 11, Chapter 10)

Solution — Step by Step

The diagonal relationship refers to the similarity in properties between certain elements of Period 2 and the elements diagonally below them in Period 3. This occurs because:

Moving across a period: size decreases, electronegativity increases
Moving down a group: size increases, electronegativity decreases

The diagonal movement combines both effects, and they roughly cancel out, resulting in similar charge density (charge/size ratio).

Lithium (Group 1, Period 2) resembles Magnesium (Group 2, Period 3) more than it resembles Na (its own group member):

Both form oxides that are not easily reduced: Li $_2$ O and MgO (unlike Na $_2$ O $_2$ , which is a peroxide)
Both carbonates decompose on heating: Li $_2$ CO $_3$ → Li $_2$ O + CO $_2$ and MgCO $_3$ → MgO + CO $_2$ . Other alkali metal carbonates are stable to heat.
Both form nitrides by direct reaction with N $_2$ : 6Li + N $_2$ → 2Li $_3$ N and 3Mg + N $_2$ → Mg $_3$ N $_2$ . No other alkali metal does this.
Both are harder and have higher melting points than other members of their respective groups.

Beryllium (Group 2, Period 2) resembles Aluminium (Group 13, Period 3):

Both have amphoteric oxides: BeO and Al $_2$ O $_3$ dissolve in both acids and alkalis. Other alkaline earth oxides are basic only.
Both form covalent compounds with significant covalent character (due to high charge density): BeCl $_2$ and AlCl $_3$ are both covalent, unlike the ionic MgCl $_2$ .
Both form complex ions with fluoride: $[\text{BeF}_4]^{2-}$ and $[\text{AlF}_6]^{3-}$ .
Both carbides react with water to give methane: Be $_2$ C + 4H $_2$ O → 2Be(OH) $_2$ + CH $_4$ , and Al $_4$ C $_3$ + 12H $_2$ O → 4Al(OH) $_3$ + 3CH $_4$ .

Why This Works

The similarity boils down to polarising power (charge/radius ratio). Li $^+$ is small with +1 charge; Mg $^{2+}$ is larger but has +2 charge. Their charge density values are similar (~1.5 and ~3.0 respectively — in the same ballpark compared to Na $^+$ at ~1.0).

Similarly, Be $^{2+}$ is tiny with +2 charge, giving it a very high polarising power that makes it behave like the trivalent Al $^{3+}$ (which is larger but has higher charge).

Alternative Method — Fajans’ Rules Connection

Fajans’ rules predict covalent character based on small cation + large anion. Both Li $^+$ and Be $^{2+}$ are very small, giving their compounds significant covalent character — just like their diagonal neighbours Mg and Al.

For NEET, the most commonly tested fact is: “Li $_2$ CO $_3$ decomposes on heating while Na $_2$ CO $_3$ does not.” This directly tests the diagonal relationship. Similarly, “BeO is amphoteric while MgO is basic” is a board exam favourite.

Common Mistake

Students often confuse diagonal relationship with group trends. The diagonal relationship is specifically between Period 2 and Period 3 elements. It does NOT extend further — for example, there is no special diagonal relationship between Na and Ca, or between Mg and Ga. The anomalous behaviour of Period 2 elements (due to their small size and absence of d-orbitals) is what makes this relationship possible.

Question

Solution — Step by Step

Why This Works

Alternative Method — Fajans’ Rules Connection

Common Mistake

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