Select the stronger bond in each pair:

(a) Cl-Cl or Br-Br

(b) Br-Br or I-I

(c) F-F or Cl-Cl.

Why doesn't the F-F bond strength follow the group trend?

The elements in the seventeenth group (7A group) of the modern periodic table are called halogens. These elements are highly electronegative with the general outer electronic configuration.

They contain 7 electrons in the valence shell and require one more electron to complete their octet. Therefore, they have high electron affinity and can form an ionic and covalent bond. The size of the halogens increases down the group while electron affinity decreases down a group.

Halogens can form covalent bonds with atoms of the same element to form diatomic species. The unpaired electron in the atom's valence shell is shared, so that both the atoms obtain an octet configuration.

All the halogens can form these bonds, and the bond's strength decreases down the group. This is because the region occupied by shared electrons becomes proportionally smaller.

As the atomic radius increases down a group, the overlapping between the orbitals becomes weaker. As a result of poor overlapping the bond become weaker as we move down the group

(a) Chlorine is in the third period, and bromine is in the fourth period. Therefore, chlorine forms a stronger bond than bromine.

Cl-Cl is the stronger bond.

(b) Bromine is in the fourth period, and iodine is in the fifth. Therefore, bromine forms a stronger bond than iodine.

Br-Br is the stronger bond.

(c) The covalent bond formed between two fluorine atoms is weak. Therefore, chlorine forms a stronger bond than fluorine

Cl-Cl is the stronger bond.

Therefore, fluorine shows a deviation from this trend. This is because the fluorine atoms are very small. Therefore, the F-F bond length is very short, and the lone pairs on fluorine experience high repulsion, making the bond unstable.