M shell of two elements A and B have 18 electrons each. The difference in the number of electrons present in N shell of A and B is 8. M shell is the penultimate shell in B. Predict the range of probable atomic numbers of A and B.

Explanation

As the penultimate shell of B is the M-shell which
possesses 18 electrons, the valence shell should be the N-shell. The possible electronic configuration of B ranges from 2, 8, 18, 1 to 2, 8, 18, 8.
The atomic numbers of B may range from Z = 29 to Z = 36.
The electronic arrangement of A corresponding to the respective arrangement of B ranges from 2, 8, 18,9, 2 to 2, 8, 18, 16, 2. The atomic number of A may range from Z = 39 to Z = 46.