Question

In the following, the number of paramagnetic molecules are: O$_2$, N$_2$, F$_2$, B$_2$, Cl$_2$.

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

A molecule is paramagnetic if it contains at least one unpaired electron in its molecular orbitals.

Answer 1

The Correct Option is C

Paramagnetism arises due to the presence of unpaired electrons in a molecule. Let’s analyze each molecule's electronic configuration to determine the number of paramagnetic molecules: 

- O$_2$ (Oxygen): Oxygen molecule has a total of 16 electrons. According to molecular orbital theory, the electronic configuration for O$_2$ is: $$(\sigma_{1s})^2 (\sigma_{1s}^*)^2 (\sigma_{2s})^2 (\sigma_{2s}^*)^2 (\sigma_{2p_z})^2 (\pi_{2p_x})^1 (\pi_{2p_y})^1 (\pi_{2p_x}^*)^0 (\pi_{2p_y}^*)^0$$ This shows that there are 2 unpaired electrons, making O$_2$ paramagnetic.

 - N$_2$ (Nitrogen): Nitrogen molecule has a total of 14 electrons. Its molecular orbital configuration is: $$(\sigma_{1s})^2 (\sigma_{1s}^*)^2 (\sigma_{2s})^2 (\sigma_{2s}^*)^2 (\sigma_{2p_z})^2 (\pi_{2p_x})^2 (\pi_{2p_y})^2$$ All electrons are paired in N$_2$, so it is diamagnetic.

 - F$_2$ (Fluorine): Fluorine molecule has a total of 18 electrons. The electronic configuration is: $$(\sigma_{1s})^2 (\sigma_{1s}^*)^2 (\sigma_{2s})^2 (\sigma_{2s}^*)^2 (\sigma_{2p_z})^2 (\pi_{2p_x})^2 (\pi_{2p_y})^2 (\pi_{2p_x}^*)^1 (\pi_{2p_y}^*)^1$$ This configuration shows 2 unpaired electrons, making F$_2$ paramagnetic.

 - B$_2$ (Boron): Boron molecule has a total of 10 electrons. Its electronic configuration is: $$(\sigma_{1s})^2 (\sigma_{1s}^*)^2 (\sigma_{2s})^2 (\sigma_{2s}^*)^2 (\pi_{2p_x})^1 (\pi_{2p_y})^1$$ This configuration shows 2 unpaired electrons, making B$_2$ paramagnetic.

 - Cl$_2$ (Chlorine): Chlorine molecule has a total of 18 electrons. The electronic configuration is: $$(\sigma_{1s})^2 (\sigma_{1s}^*)^2 (\sigma_{2s})^2 (\sigma_{2s}^*)^2 (\sigma_{2p_z})^2 (\pi_{2p_x})^2 (\pi_{2p_y})^2 (\pi_{2p_x}^*)^1 (\pi_{2p_y}^*)^1$$ This configuration shows 2 unpaired electrons, making Cl$_2$ paramagnetic. Thus, the paramagnetic molecules are O$_2$, F$_2$, and B$_2$. Therefore, the number of paramagnetic molecules is 3.