Correct Answer:

Innermost Shell - 2 electrons

Middle shell - 8 electrons

Outermost Shell - 0 electrons

Magnesium 26 is categorized as an alkaline earth metal and falls in group 2 of the periodic table. This class of elements generally will "give up" their valence electrons when they enter an excited state of the correct energy. An excited state is any state higher than the ground state. According to the modern wave mechanical model electrons in a lower orbital will move to a higher orbital when provided with sufficient energy (excited). This is in contrast to the non-metal group of elements, which are very reactive and will receive electrons in there outermost orbital.

Answering the question:

In order to answer the question it helps to know the groups of elements in the periodic table, ranging from metals to non-metals. The first two columns are the alkali metals and alkaline earth metals. There groups have the tendency to give up there valence electrons when excited sufficiently. This would result in the valence electrons receiving sufficient energy and move to a high enough orbital such that the nucleus no longer possesses sufficient attractive force to hold the valence electrons in orbital.

The number of valence electrons present is indicated by the group in which the elements occur. Since magnesium (Mg) is a member of the metals group we know it will have the tendency to give up its electrons, based on the diagram there are two electrons in its valence orbital. The result would be magnesium with two more protons than electrons and hence a net positive charge represented by Mg²⁺. The electrons of the two inner orbitals would be the only remaining electrons resulting in the configuration in the answer above.