The reactivity of an atom arises from

a. the average distance of the outmost electron shell from the nucleus.

b. the existence of unpaired electrons in the valance shell.

c. the sum of the potential energies of all the electron shells.

d. The potential energy of the valance shell.

B

Explanation:

For metals, the further away the valence electrons are from the nucleus, the higher the reactivity of the elements. Unlike halogens that are more reactive if the valence orbital shells are closer to the nucleus. This is due to ionization energies - which is the minimum energy required to discharge an electron from its orbit.

For metals this should be lower because they need to lose electrons to attain a stable electron configuration. For halogens it should be high because they don't need to lose electrons, but rather gain, in order to attain stable electron configuration.

Electrons in their orbital shells need to occur in pairs (with opposite quantum states) to be in stable configuration and as long as an atom has orbitals with single electrons, the atoms will be reactive as opposed to when its orbitals are 'full'.