The overlap of s and p Z orbitals of H and Cl, respectively leads to the formation of the covalent bond in HCl.
Step-by-step explanation
According to valence bond theory, the elements combine to form a compound by overlapping of orbitals with half filled electrons in it.
The valence bond theory is mostly suitable for compounds formed by covalent bonding as the valence bond theory assumes the overlapping or sharing of half filled electrons in the outermost shell.
In HCl, the electronic configuration of H is 1s1 while the electronic configuration of Cl is [Ne] 3s23p5.
So the outermost shell or the valence shell of the elements H and Cl contains 1 and 7 number of electrons.
So the 1 electron in the H will be shared with the 1 unpaired electron in the p orbital of the Cl element so that both H and Cl will be stabilized.
It is known that p orbital will orient in px,py and pz orbitals.
So the 5 electrons in the p orbital of Cl will be divided as 2 electrons in px sub-orbital, 2 electrons in py sub-orbital and remaining 1 unpaired electron will be occupied in pz sub-orbital.
The arrangement of electrons in the p subshell is done with respect to Pauli's exclusion principle and Aufbau principle.
Thus the unpaired electron of hydrogen when is in s shell will overlap with the unpaired electron in pz sub-orbital of chlorine to form HCl compound.
So the overlap of spz orbitals lead to formation of HCl compound.