Why is an atom with fewer electrons in the outer shell more reactive?

That isn't really true. What makes atoms more reactive is if their electronic configuration is nearly that of a noble gas -- in other words is either has just one or two electrons less or more than a noble gas.So elements with either 1, 2, 6 or 7 valence electrons tend to be especially reactive.

so the guy who gave the first answer doesnt know what hes talking about.