Question

Among the following, which element has the same number of electrons present in both K-shell and M-shell?

• A. Helium
• B. Beryllium
• C. Magnesium
• D. Carbon

Answer 1

The correct option is C Magnesium

Magnesium has same number of electrons in its K-shell and M-shell of the atom.
Atomic number of magnesium ($\text{Mg}$) is 12 and hence its electronic configuration is (2, 8, 2). This means this element has 2 electrons in its K-shell, 8 electrons in L-shell and 2 electrons in its M-shell of the atom.

Electronic configuration of these elements along with their atomic numbers are given below:

$$\begin{array}{ccc}\text{Element}& \text{Atomic number}& \text{Electronic configuration (K,L,M)}\\ \text{Helium}& 2& \text{(2)}\\ \text{Beryllium}& 4& \text{(2, 2)}\\ \text{Carbon}& 6& \text{(2, 6)}\\ \text{Magnesium}& 12& \text{(2, 8,2)}\end{array}$$