The atomic number of element A is 36 and that of B is 19, by representing the atom in electronic configuration,

For A:

K = 2

L = 8

M = 18

N = 8

The outermost shell N is complete with the octet so the element is stable in this state and will not be reactive because of the 0 valency and complete octet in the atom.

For B:

K = 2

L = 8

M = 9

The valence electron in the element is 9 so it is not that stable, it will readily lose 1 electron in order to complete its octet of electrons and become stable.

So element B will be more reactive than the element A because of the complete octet in element A.