when an unpaired orbit acquires a fill electron, it is the proton that attracts the negative particle. Although it is just as probable that the positive side of the weak field acquires the negative particle.
The likelihood of each scenario depends on which configuration has lower energy. So, it is possible that,
when the orbit is unpaired, the negative particle jumps from orbit around the proton into the weak field with an release of energy, and vice versa.
When the work function is not half the bond energy, the negative particle is first acquired by the weak field. When a passing photon unpairs the orbits temporarily, it is possible that the electron jumps into weak field with an release of energy before the orbits pair up again with another release of energy.
And that's how the metal gets heated up.
Note: Within the metal, may or may not be covalent bond as we know it.
