Question

An atom of an element has two electrons in the M shell. If M is the outermost shell, what is the atomic number of that element?

• A. 2
• B. 10
• C. 12
• D. 8

Answer 1

The correct option is C 12

The order of energy levels of shells in an atom is K, L, M, N, etc., and according to the Bohr-Bury scheme: the maximum number of electrons in a particular shell is given by $2n^2$. Since here M shell is the outermost shell, it means that the inner shells are completely filled, i.e. K shell has 2 electrons and the L shell has 8 electrons. The M shell comes after the L shell and it has only 2 electrons.
Therefore, the total number of electrons in the atom of the element = 2 + 8+ 2
= 12 electrons
Since the atom is neutral,
Atomic number of the element = total no. of protons = total number of electrons = 12