Click on the diagram to choose which atom is more reactive. Hint: These are nonmetals. Which is going to gain a full valence shell of 8 electrons the easiest?

The first one has 5 valence electrons, and the second has 7. (This can be counted by counting the amount of black dots on the biggest circle). Since 7 > 5, it would be easier for the second one to gain one (so that it completes it's valence shell) so the answer is the second one.