Question

Trihalides are less covalent than pentahalides. Justify

Answer 1

Fajan's Rule

• The covalent character depends on the polarization.
• "More the polarization, the larger the covalent character of the bond.”

In the case of halides, trihalides are less covalent than pentahalides because:

Trihalides

• Trihalides $\left({\mathrm{MX}}_3\right)$and Pentahalides $\left({\mathrm{MX}}_5\right)$are formed when group $15$elements are reacted with a halogen atom.
• All the Group $15$elements form trihalides.
• Because of the $+3$oxidation state, the central atom polarizes the halogen atom to a lesser extent in trihalides than in pentahalides.
• More the polarization, the larger the covalent character of the bond.
• So Trihalides are less covalent.

Pentahalides

• All the Group $15$elements form pentahalides. Except for Nitrogen.
• Nitrogen does not form pentahalides because it does not have a vacant d-orbital in its outermost shell.
• Because of the $+5$oxidation state, the central atom polarizes the halogen atom to a greater extent in trihalides than in pentahalides.
• More the polarization, the larger the covalent character of the bond.
• So Pentahalides are more covalent.

As a result of the greater polarization of the bond in the pentahalide state as compared to the trihalide state, thus, trihalides are less covalent than pentahalides.