Question

Which of the following atom has no neutron in its nucleus.

• A. Hydrogen
• B. Nitrogen
• C. Helium
• D. Boron

Answer 1

The correct option is A

Hydrogen

Explanation for correct option:

A. Hydrogen

• Hydrogen is the chemical element with the symbol H and atomic number $1$.
• Hydrogen is the lightest element.
• The electronic configuration of hydrogen is $1s^1$.
• The hydrogen atom does not contain neutron, because its nucleus is the smallest in size and cannot accommodate any heavier neutron which makes it unstable.

Explanation for incorrect options:

B. Nitrogen has an atomic number $7$ and its electronic configuration is $1s^{2}2s^{2}2p^3$ and it has $7$ neutrons.

$$\begin{gathered} \mathrm{Number}\mathrm{of}\mathrm{neutrons}=\mathrm{Mass}\mathrm{number}-\mathrm{Number}\mathrm{of}\mathrm{protons} \\ \mathrm{Number}\mathrm{of}\mathrm{neutrons}=14-7=7 \end{gathered}$$

C. Helium has an atomic number $2$ and its electronic configuration is $1s^2$ and it has $2$ neutrons.

$$\begin{gathered} \mathrm{Number}\mathrm{of}\mathrm{neutrons}=\mathrm{Mass}\mathrm{number}-\mathrm{Number}\mathrm{of}\mathrm{protons} \\ \mathrm{Number}\mathrm{of}\mathrm{neutrons}=4-2=2 \end{gathered}$$

D. Boron has an atomic number $5$ and its electronic configuration is $1s^2\mathit{2}{s}^{\mathit{2}}\mathit{2}{p}^{\mathit{1}}$ and it has $5$ neutrons.

$$\begin{gathered} \mathrm{Number}\mathrm{of}\mathrm{neutrons}=\mathrm{Mass}\mathrm{number}-\mathrm{Number}\mathrm{of}\mathrm{protons} \\ \mathrm{Number}\mathrm{of}\mathrm{neutrons}=10-5=5 \end{gathered}$$

Hence, the correct option is A i.e. Hydrogen.