Question

The maximum covalency of Boron and Aluminium respectively are

• A. 3,4
• B. 3,6
• C. 4,6
• D. 4,4

Answer 1

Answer: (C)

4,6

Answer 2

• Boron (B):

  • Electronic configuration: $1s^2 2s^2 2p^1$.
  • Valence shell has one 2s orbital and three 2p orbitals.
  • In the excited state, Boron can have one electron in 2s and two electrons in 2p, leading to 3 unpaired electrons, forming compounds like $BF_3$ (covalency 3).
  • Boron has a vacant 2p orbital. It can accept a lone pair of electrons from a donor atom (e.g., $F^-$) to form a dative bond, forming species like $BF_4^-$.
  • In $BF_4^-$, Boron forms 4 bonds (3 covalent + 1 dative).
  • The maximum number of orbitals available in the second shell (n=2) is four (one 2s and three 2p). Thus, Boron can accommodate a maximum of 8 electrons (octet rule) and its maximum covalency is 4. It cannot expand its octet due to the absence of d-orbitals.

• Aluminium (Al):

  • Electronic configuration: $1s^2 2s^2 2p^6 3s^2 3p^1$.
  • Valence shell has one 3s orbital, three 3p orbitals, and five vacant 3d orbitals.
  • Similar to Boron, Aluminium can form 3 covalent bonds (e.g., $AlCl_3$).
  • Due to the presence of vacant 3d orbitals, Aluminium can expand its octet and accommodate more than 8 electrons in its valence shell.
  • It can accept lone pairs of electrons into its vacant d-orbitals to form complex ions. For example, in $[AlF_6]^{3-}$ or $[Al(H_2O)_6]^{3+}$, Aluminium forms 6 bonds.
  • The hybridization in such complexes is $sp^3d^2$, utilizing one 3s, three 3p, and two 3d orbitals, leading to a maximum covalency of 6.

Therefore, the maximum covalency of Boron is 4, and that of Aluminium is 6.