Question

The first two orbits of an atom are completely filled with electrons and no electron is there in the third shell. The atomic number of the element is ________.

• A. 18
• B. 2
• C. 10
• D. 32

Answer 1

The correct option is C 10

- Maximum number of electrons that a shell can accommodate is given by $2n^2$. The table given below shows the maximum number of electrons present in first three in shells.$$\begin{array}{ccc}\text{Shell}& \text{Shell number, n}& \text{Maximum number of electrons}\\ \text{K}& \text{1}& \text{2}\\ \text{L}& \text{2}& \text{8}\\ \text{M}& \text{3}& \text{18}\end{array}$$ - As the first two shells are completely filled the number of electrons present in the element is 10.
- The atomic number of a neutral element = number of protons = number of electrons present in the orbits
- Hence, the atomic number of the element is 10.