See You Not

It is likely that the unpaired orbit with an unbalanced weak field extend further than the other paired orbits.  The filled paired orbits shield the lone proton from the electron at the positive end of the holding weak field.

The proton acquires another electron and \(Cl\) gains a negative charge to become \(Cl^{-}\).

Speculative as always...

In general, nucleus with many paired orbits can shield unpaired orbits from their filled electron and allow them to acquire another electron in their immediate vicinity.  This results in an anion.

Note: The other case where the weak field holds two electrons is ruled out because of the strong repulsion between the electrons.  In the above case, the extra electron is held by the proton and repulsed by the weak field.  Shielding reduces the weak field further.  It is likely that the lone electron held by the weak field, be within the shielding provided by the filled paired orbits.