How many electrons do most atoms seek to have on their valence shell? Using this information, explain why certain elements are unreactive and why other elements are reactive.

For most elements, having 8 valence electons makes them very stable; The lower the valence electrons an element has, the lower is its stability. Inversely, the higher its reactivity.  

Explanation:

Most atoms needs 8 valence electrons to obtain "a happy state." For most elements, 8 valence electrons makes them very stable. In fact this is why, elements found in group 8A, the noble gases, are very stable: they already have their valence shell completely filled. Helium is the only noble gas that needs 2 atoms in order to be stable. On the left side of the periodic table we can find group 1A and 2A. These are both very unstable groups. The most unstable elements are found in 1A the lower you go down the periodic table. This is because these elements have high ionization energy. This energy basically tells us how much the elements want to lose their valence electrons (for bonding). Additionally, if 8 electons are required to fill a valence shell, then 1 or 2 electrons (found in groups 1a & 2A) are certainly far from being stable. The lower the valence electrons an element has, the lower is its stability, thus the higher is its reactivity.

the potential energy of the reactants is 200j.

from the energy diagram, the energy of the product formed is 350j; this means that, this reaction is an endothermic reaction, because it absorbs energy from its environment.