The metal with which hydrogen reacts to form metal hydride is:
A. Hg
B. Ca
C. Ag
D. H

Hint: The metal which reacts with hydrogen form metal hydride is an alkaline earth metal. The atomic number of the metal is 20. It belongs to group 2 elements and period 4.

Complete step by step answer: Metal hydrides are defined as the compounds which are formed when an metal element is bonded to a hydrogen. In metal hydride, the metal is present in the form of cation and hydrogen is present in the form of anion. In metal hydride, the oxidation state of hydrogen is -1. The hydrogen anion acts as a nucleophile in metal hydride compounds.
Mercury reacts with hydrogen to form mercury(I) hydride with chemical formula HgH where the oxidation state of mercury is +1 and oxidation state of hydrogen is -1. Mercury is a cation and hydrogen is an anion. But mercury(I)hydride is a very unstable compound and is not obtained in bulk so it is excluded.
Calcium reacts with hydrogen to form calcium hydride with chemical formula $Ca{H_2}$ where the oxidation state of calcium is +2 and oxidation state of hydrogen is -1. The calcium is a cation and hydrogen is the anion.
Silver does not react with hydrogen to form hydride compounds as it is insoluble in water at normal condition.
Thus, the metal with which hydrogen reacts to form metal hydride is calcium (Ca).
So, the correct answer is “Option B”.

Note:
The elements of group 7, 8 and 9 show low affinity towards hydrogen atoms, therefore they do not form metal hydride. This property is commonly known as the hydride gap in the periodic table.