Reactivity of an element is determined by its tendency to undergo chemical reactions. Nickel, with an atomic number of 28, belongs to the transition metal group in the periodic table. Transition metals are generally more reactive than non-metals. On the other hand, Xenon, with an atomic number of 54, is a noble gas. Noble gases are known for their exceptional stability and low reactivity due to their complete electron shells.

In terms of atomic structure, Nickel has a partially filled 3d electron subshell, which makes it prone to participate in chemical reactions to achieve a stable electron configuration. Xenon, however, has a completely filled 5p electron subshell, making it highly stable and less likely to undergo chemical changes.

Therefore, Nickel, being a transition metal with partially filled electron subshells, is more reactive compared to Xenon, which is a noble gas with a stable electron configuration.