Which orbital overlap produces a σ bond in HCl?

The electronic configuration of H is $1s^1$.

The electronic configuration of Cl is $1s^2 2s^2 2p^6 3s^2 3p^5$.

The unpaired electron in the hydrogen atom is present in the $1s$ orbital, which is spherically symmetrical.

The unpaired electron in the chlorine atom is present in one of the $3p$ orbitals, which is dumbbell-shaped.

The formation of the HCl molecule involves the axial overlap of the half-filled $1s$ orbital of the hydrogen atom with the half-filled $3p$ orbital of the chlorine atom.

This head-on overlap between an $s$ orbital and a $p$ orbital results in the formation of a $\sigma$ bond.

Thus, the orbital overlap that produces a $\sigma$ bond in HCl is $s-p$.

Answer: s$-$p