Question

Which of the following elements have two electrons in its K shell?

• A. Hydrogen
• B. Helium
• C. Sodium
• D. Aluminium

Answer 1

Answer: (B), (C), (D)

The correct options are
B Helium
C Sodium
D Aluminium

$\bullet$ The maximum electrons that a shell can accommodate is given by $2n^2$, where n is the shell number.

$\bullet$ So, the maximum numbers of electrons that K, L and M shells can accommodate are 2, 8 and 18 respectively.
$$\begin{array}{ccc}\text{Element}& \text{Atomic No.}& \text{Electronic Configuration}\\ \text{Hydrogen}& \text{1}& \text{1}\\ \text{Helium}& \text{2}& \text{2}\\ \text{Sodium}& \text{11}& \text{2,8,1}\\ \text{Aluminium}& \text{13}& \text{2,8,3}\end{array}$$
$\bullet$ Hence, all the atoms except hydrogen have two electrons in its K shell.