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.