Bond energy generally decreases on moving
from top to bottom along a group. It happens due to fact
that size increases on moving down the group and
thus, the two nuclei are far apart and less capable
of holding the two atoms together.
But in case of F2 molecule due to more repulsion in between non-bonding electron pair (2p) of two fluorines (due to small size of F-atom) in comparison to non-bonding electron pair (3p) in chlorine, the bond energy of F2 is less than Cl2.
BE(F2) = 158.5 kJ/mole and
BE(Cl2)= 242.6 kJ/mole
Questions Asked from Periodic Table and Periodicity
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