A metal which does not react with nitrogen is?
A. Li
B. K
C. Ca
D. Mg

Hint: Nitrogen is a non-metal. So it usually reacts with metals to form ionic compounds. The reaction can occur only if the reactants end up forming products such that energy is released by the system as a whole. If the energy is needed by the system to undergo the reaction, then the reaction cannot occur.

Complete step by step solution:
-Exothermic reactions are the reactions in which the energy is released into the system. In this type of the reaction, the overall sum of the enthalpies of the reactants is greater than that of the product. As a result, the net change in enthalpy comes out to be negative in exothermic processes.
-Endothermic reactions are the reactions in which the energy is absorbed by the system. In this type of the reaction, the overall sum of the enthalpies of the reactants is lesser than that of the product. As a result, the net change in enthalpy comes out to be positive in endothermic reactions.
- Whenever a reaction occurs so as to form a compound, energy absorbed is the ionization energy and energy released is the electron affinity as well as the lattice energy. Compounds are formed when the sum of lattice energy and electron affinity is more than the sum of all the ionization energies of the atoms.
-It means that if the reaction of nitrogen with metals is exothermic, then only can the products be formed. If the overall reaction turns out to be endothermic, then the reaction of nitrogen with that metal is not considered to be feasible.
-Nitrogen reacts with lithium, calcium and magnesium to form compounds such that energy is released by the system. It means that these reactions are exothermic. So, these reactions are feasible.
- The reaction of nitrogen with the other metals can be sown as
\[\begin{align}
  & 6Li+{{N}_{2}}\to 2L{{i}_{3}}N \\
 & 3Mg+{{N}_{2}}\to M{{g}_{3}}{{N}_{2}} \\
 & 3Ca+{{N}_{2}}\to C{{a}_{3}}{{N}_{2}} \\
\end{align}\]

-When nitrogen reacts with potassium, then the overall energy is absorbed by the system. This is because the lattice energy and the electron affinity of nitrogen do not exceed the ionization energy of potassium. So the overall reaction is endothermic and so infeasible.

Therefore the correct option is B.

Note: The sum of the ionization energies of the atoms are taken into account. If the atom belongs to group 1. Then only the first ionization energy is taken. If the atom is of group 2, then the sum of first and second ionization energies are taken. If the atom is of group 3, then the sum of first, second and third ionization energies are taken into account. Same goes with the bond dissociation enthalpy.