Question

Which of the following molecules is paramagnetic?

• A. \(\text{Li}_2\)
• B. \(\text{N}_2\)
• C. \(\text{O}_2\)
• D. \(\text{F}_2\)

Hint:

Write the MO configuration and look for unpaired electrons.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
A molecule is paramagnetic when its molecular orbital configuration has at least one unpaired electron.

Step 2: Key Formula or Approach:
Fill the MO energy levels. For \(\text{O}_2\) and \(\text{F}_2\), the order is \(\sigma 2p_z\) below \(\pi 2p\).

Step 3: Detailed Explanation:
\(\text{Li}_2\): \(\sigma 1s^2\,\sigma^*1s^2\,\sigma 2s^2\). All paired, diamagnetic.
\(\text{N}_2\): \(\ldots\pi 2p_x^2\,\pi 2p_y^2\,\sigma 2p_z^2\). All paired, diamagnetic.
\(\text{O}_2\): 16 electrons, with the last two going singly into the two degenerate \(\pi^*2p_x\) and \(\pi^*2p_y\) orbitals. Two unpaired electrons, so paramagnetic.
\(\text{F}_2\): the \(\pi^*\) orbitals are completely filled. All paired, diamagnetic.

Step 4: Final Check:
Only \(\text{O}_2\), which is option (C), has unpaired electrons.

Final Answer:
Only O2 has unpaired electrons in its MO diagram. \[ \boxed{\text{(C) }\text{O}_2} \]