Answer:
Magnesium
Step-by-step explanation:
Reactivity is based on valence shell electrons.
Valence Shell Electrons
Valence shell electrons are electrons that are on the outermost shell of an atom. For the major periodic families, you can look at their family numbers to see how many valence electrons they have. Magnesium is in family 2, which all have 2 valence electrons. Nitrogen is in family 15, which has 5 valence electrons.
Reactivity
The octet rule states that atoms are more stable when they have full valence shells (8 valence electrons). So, atoms will attempt to lose or gain electrons in order to fulfill the octet rule. An atom can be stable by losing all of its valence electrons or by having 8.
Magnesium has to lose 2 valence electrons and nitrogen has to gain 3 valence electrons to fill the octet. The closer an atom is to filling the octet, the more reactive it will be. Since magnesium is closer to the octet, it is more reactive than nitrogen.
However, elements such as lithium (1 valence electron) or fluorine (7 valence electrons) that are only 1 electron away from filling the octet will be the most reactive.