First slide

Electronic configuration and Atomic , physical properties

Question

Match the correct bond dissociation in halogens
   Halogens       Bond energy
   1) F₂       A) 158.8 KJmol-1
   2) Cl₂        B) 242.6KJmol-1
   3) Br₂       C) 151.1KJmol-1
   4) I₂        D) 192.8 KJmol-1

Easy

A

1 – A, 2 – B, 3 – D, 4 – C
B

1 – A, 2 – B, 3 – C, 4 – D
C

1 – B, 2 – A, 3 – D, 4 – C
D

1 – D, 2 – C, 3 – A, 4 – B

Solution

The decreasing order of bond energies of halogens is: Cl₂ > Br₂ > F₂ > I₂.
The exceptionally lower bond energy of Fluorine is due to Fluorine atom is very small.
The inter nuclear distance in fluorine is only 1.44 A°. Fluorine has notoriously weak bond.
This was explained successfully by Coulson.
The non-bonding electron pairs of adjacent fluorine atoms in F₂ molecule are very close to each other.
These lone pairs of electrons are under strong repulsive forces. Hence the bond strength in fluorine is greatly reduced.

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