Question

Strongest bond is formed by the head-on overlapping of:
A.2s and 2p orbitals
B.2p and 2p orbitals
C.2s and 2s orbitals
D.All of these

Answer 1

Recall the valence bond theory which states that bonds are formed by the overlap of atomic orbitals. The strength of the bond is dependent on the extent of the overlap. Individual bonds formed can be sigma or pi bonds. But here it is mentioned head-on overlap so, a sigma bond will be formed.

Complete answer:
As we know, a sigma bond is formed by head-on or end-to-end overlapping. Also, the directional character of the orbital is taken into account. Thus, the strength of a sigma bond is directly proportional to the extent of overlapping and the directional character.
Here we have two orbitals s and p. The ‘s’ orbital has a spherical symmetry hence it tends to have a uni-directional character. However, the ‘p’ orbital has a multi-directional character; it can align itself in the x, y, and z-axis. Thus, the overlapping between s and s orbital would cover less area and so its strength will be less.
On the other hand, the area for p and p orbital overlapping will be more which will eventually increase the strength of the bond.
Also, overlapping between P orbital and s orbital will be more than s-s overlap but less than p-p overlap.
So the increasing order of strength of Sigma bond will be:
$s-s$< $s-p$<$p-p$
During the axial overlap of p-p orbitals, the electron density increases around the axis, so the bond formed is the strongest. Therefore, the strongest bond formed is when p-p orbital overlap occurs.
Final answer: The correct answer is Option B- 2p and 2p orbitals.

Note:
Pi bonds are formed when the orbital overlap occurs between parallel p orbitals and the sigma bond is formed by the axial overlap of the p orbital. Also, the p orbital (Pz-Pz) that overlaps head-on has more electrostatic attraction due to its orientation in space.