s-Block Elements - Anomalous Properties of Lithium and Beryllium

Lithium ($Li$) and Beryllium ($Be$) are the first elements of Group 1 and Group 2 respectively, and they exhibit properties that differ significantly from the rest of their groups due to their exceptionally small atomic and ionic sizes.

The anomalous behavior is attributed to: (i) Small size of atom and ion, (ii) High electronegativity and high ionization enthalpy, (iii) High polarizing power (Charge/Radius ratio), and (iv) Absence of $d$-orbitals in their valence shell.

Lithium's polarizing power is high, leading to a significant covalent character in its compounds (e.g., $LiCl$ is soluble in organic solvents).

Lithium is the only alkali metal which reacts directly with nitrogen to form a nitride: $6Li + N_2 \rightarrow 2Li_3N$.

Lithium carbonate ($Li_2CO_3$) and lithium nitrate ($LiNO_3$) are thermally unstable; $LiNO_3$ decomposes to give $Li_2O$, $NO_2$, and $O_2$, while other alkali metal nitrates give nitrites and $O_2$.

Beryllium forms compounds that are largely covalent, such as $BeCl_2$ which has a polymeric chain structure in the solid state.

Beryllium oxide ($BeO$) and beryllium hydroxide ($Be(OH)_2$) are amphoteric, reacting with both acids and bases, whereas oxides/hydroxides of other Group 2 elements are basic.

Diagonal Relationship: Lithium shows similarities with Magnesium ($Mg$) of Group 2, and Beryllium shows similarities with Aluminum ($Al$) of Group 13 due to similar ionic sizes and charge/radius ratios.

The maximum coordination number for $Be$ is 4 due to the presence of only four available orbitals (one $2s$ and three $2p$), whereas other members can have higher coordination numbers using $d$-orbitals.