Q.First member of each group of representative elements (i.e., s and p-block elements) shows anomalous behaviour. Illustrate with two examples.

Concept: Anomalous behaviour of the first element in a group arises from its small size, high electronegativity, absence of d-orbitals, and a tendency to form $p\pi$-$p\pi$ multiple bonds.

Reasoning:

1. Lithium in Group 1: Unlike other alkali metals, lithium reacts directly with nitrogen to form lithium nitride ($6\text{Li} + \text{N}_2 \rightarrow 2\text{Li}_3\text{N}$). This is due to its small size and high lattice energy of the nitride. No other alkali metal forms a stable nitride at room temperature. …

The concept behind this anomaly is rooted in the electronic configuration and atomic structure of the first element in a group. Unlike its heavier counterparts, the first member has no inner d- or f-electrons, a much smaller atomic radius, and a higher charge-to-size ratio. This makes it behave more like the element diagonally below it in the next group (diagonal relationship) and often form covalent compounds where others form ionic ones. Let’s break this down with two clear examples.

1. Lithium (Li) in Group 1 (alkali metals)

   Lithium is the first member of Group 1. While sodium, potassium, and other alkali metals are highly reactive metals that form ionic compounds readily, lithium shows several differences.

   • Small size and high polarising power: Li⁺ has a very small radius (76 pm) compared to Na⁺ (102 pm). This gives it a high charge density, so it polarises anions strongly, making its compounds more covalent. For example, LiCl is soluble in organic solvents like ethanol, while NaCl is not.
   • Diagonal relationship with magnesium: Lithium resembles magnesium (Group 2) more than its own group. Both form nitrides when heated in nitrogen (Li₃N and Mg₃N₂), while other alkali metals do not. Both also form oxides (Li₂O and MgO) rather than peroxides or superoxides.
   • Stability of lithium carbonate: Li₂CO₃ decomposes on heating to Li₂O and CO₂, like MgCO₃, whereas Na₂CO₃ is thermally stable. This anomalous behaviour is why lithium is stored in oil but reacts less violently with water than sodium.

2. Boron (B) in Group 13 (boron family)

   Boron is the first member of Group 13. While aluminium, gallium, and indium are metals, boron is a metalloid with entirely different chemistry.

   • Covalent nature: Boron forms covalent compounds (e.g., BF₃, BCl₃) because its small size and high ionisation energy prevent it from losing three electrons to form B³⁺. Aluminium, in contrast, can form ionic Al³⁺ compounds like Al₂O₃.
   • Electron deficiency: Boron has only three valence electrons but needs four to complete an octet. This makes it electron-deficient, so compounds like BF₃ act as Lewis acids, accepting electron pairs. Aluminium compounds also show Lewis acidity but less prominently. …