The S-Block Elements (Anomalous Behaviour Of Beryllium)

Anomalous Behaviour Of Beryllium

Beryllium (Be), the first element of Group 2, exhibits anomalous behavior compared to the rest of the alkaline earth metals. This is primarily due to its small atomic and ionic size, high ionization enthalpies (highest among Group 2), and high charge density.

Diagonal Relationship Between Beryllium And Aluminium

Beryllium shows a diagonal relationship with Aluminum (Al), the second element of Group 13. This similarity in properties arises because of the comparable charge density ($charge/size$ ratio) of $Be^{2+}$ and $Al^{3+}$ ions.

Similarities in Ionic Size and Charge Density:

• Ionic Radii: $Be^{2+}$ (radius $\approx$ 31 pm) and $Al^{3+}$ (radius $\approx$ 53.5 pm) have similar charge densities (charge/radius ratio). Although $Al^{3+}$ is larger, its charge is higher, resulting in a comparable polarizing power and charge density to $Be^{2+}$.

Similarities in Properties:

1. Hardness: Beryllium and Aluminum are harder than other elements in their respective groups.
2. Reactivity with Air: Both form a stable protective oxide layer ($BeO$ and $Al_2O_3$) on their surface which prevents further reaction with air or water.
3. Reaction with Acids: Both react with dilute acids to liberate hydrogen gas, but their reaction is slower than alkali metals or other alkaline earth metals.
4. Reaction with Bases: Both beryllium and aluminum are amphoteric. They react with strong bases to form complex salts and liberate hydrogen gas.
• $Be(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow Na_2[Be(OH)_4](aq) + H_2(g)$ (Beryllate ion)
• $2Al(s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2Na[Al(OH)_4](aq) + 3H_2(g)$ (Aluminate ion)
5. Formation of Chlorides: Both beryllium chloride ($BeCl_2$) and aluminum chloride ($AlCl_3$) are covalent, soluble in organic solvents, and exist as polymeric structures in the solid state and as dimers in the vapor phase.
• $BeCl_2$ exists as a linear polymer in solid state and dimer ($Al_2Cl_6$) in vapor phase.
• $AlCl_3$ exists as a dimer ($Al_2Cl_6$) in vapor phase and has a lattice structure in solid state.
6. Hydrolysis of Salts: Salts of beryllium and aluminum undergo extensive hydrolysis, giving acidic solutions due to the formation of complex ions like $[Be(H_2O)_4]^{2+}$ and $[Al(H_2O)_6]^{3+}$.
7. Oxides: Both beryllium oxide ($BeO$) and aluminum oxide ($Al_2O_3$) are high melting point solids and exhibit amphoteric behavior.
8. Carbonates: Both lithium (from Group 1) and beryllium ($BeCO_3$) form basic carbonates and their normal carbonates are sparingly soluble in water and decompose readily on heating.

Points of Difference: Despite the similarities, some differences exist, such as beryllium forming covalent compounds more readily due to its smaller size and higher electronegativity, whereas aluminum compounds are more ionic in nature, although $AlCl_3$ shows covalent character.