Q9.84CP - Chemistry: Molecular Nature Of Matter And Change

Question

The HF bond length is 92pm, 16% shorter than the sum of the covalent radii of H (37pm) and F(72pm). Suggest a reason for this difference. Similar calculations show that the difference becomes smaller down to the group from HF and HI. Explain

Step-by-Step Solution

Answer

Due to the more electronegativity difference, the bond length decreases.

The bond length from HF to HI can be shown as

HF > HCl > HBr > HI

Definitions

Bond length is a measure of the distance between the nuclei of two chemically bonded atoms in a molecule. 

 

Covalent radius is one-half of the distance between two nuclei of two covalently bonded atoms.

bond length

 Fluorine is the most electronegative element and is smaller in size. Hence when it forms a bond with hydrogen it attracts shared electrons more than hydrogen. Due to this, the sum of the HI bond length decreases than the sum of the bond length.

 

Down to a group, the number of electronic shells increases. Therefore, the fluorine to iodine number of shells increases, and the distance between the valence shell and nucleus decreases. Since, when iodine forms a bond with hydrogen it attracts electrons more than fluorine in an HF bond. So, the order of the difference in bond length of halogen acids is –

 

HF > HCl > HBr > HI