Examine the short-hand electron configuration of all of the elements in Period 2 (Li, Be, B, C, N, O, F and Ne). As you travel across the period, are electrons being added to the same or different energy levels (recall that the coefficient represents the energy level)? Considering that protons are added along with electrons as you move from element to element from left to right, propose an explanation for the periodic trend in atomic radius that includes the added protons and the energy level to which valence electrons are added.