(b) Anomalous Properties of Second Period Elements

Anomalous Properties of Second Period Elements

The first element of each group — lithium (group 1), beryllium (group 2), and boron through fluorine (groups 13–17) — differs significantly from the other members of its group.

Examples of Anomalous Behaviour

Lithium, unlike other alkali metals, forms compounds with pronounced covalent character. Similarly, beryllium, unlike other alkaline earth metals, also forms covalent compounds. The other members of these groups predominantly form ionic compounds.

Notice how closely lithium's metallic radius (152 pm) tracks magnesium's (160 pm) rather than sodium's (186 pm) — and beryllium's (111 pm) sits far closer to aluminium's (143 pm) than to magnesium's own value. This is the numerical evidence behind the diagonal relationship.

Reasons for Anomalous Behaviour

Four factors contribute to the anomalous behaviour of the first member of each group:

1. Small size: The first element has the smallest atomic radius in its group.
2. Large charge/radius ratio: The ratio of nuclear charge to atomic radius is highest for the first element.
3. High electronegativity: The first element is the most electronegative in its group.
4. Limited valence orbitals: The first member has only four valence orbitals ($2s$ and $2p$) available for bonding.

Example: Boron can only form $\mathrm{BF_4^-}$, where it has four bonds. Aluminium, however, can form $\mathrm{AlF_6^{3-}}$, where it has six bonds because it can use its $3d$ orbitals.

Multiple Bond Formation

The first member of p-block elements displays a greater ability to form $p\pi - p\pi$ multiple bonds:

• To itself: $\mathrm{C=C}$, $\mathrm{C\equiv C}$, $\mathrm{N=N}$, $\mathrm{N\equiv N}$
• To other second period elements: $\mathrm{C=O}$, $\mathrm{C=N}$, $\mathrm{C\equiv N}$, $\mathrm{N=O}$ …