Which of the following species contains an equal number of $\sigma$ and $\pi$ bonds?

Question

Accepted Answer

1. **Analyze $XeO_4$:** Xenon (Xe) has 8 valence electrons. It forms double bonds with four oxygen atoms to achieve a stable tetrahedral geometry. Each double bond consists of 1 $\sigma$ bond and 1 $\pi$ bond. 
   - Total $\sigma$ bonds = 4
   - Total $\pi$ bonds = 4
   - **Result:** Equal number of $\sigma$ and $\pi$ bonds .

2. **Analyze other options:**
   - **$HCO_3^-$ (Bicarbonate ion):** The structure is $HO-C(=O)-O^-$. It contains 4 $\sigma$ bonds (C-O, C=O, C-O, O-H) and 1 $\pi$ bond (in C=O). ($4\sigma, 1\pi$)
   - **$(CN)_2$ (Cyanogen):** The structure is $N \equiv C - C \equiv N$. It contains 3 $\sigma$ bonds (one C-C, two C-N) and 4 $\pi$ bonds (two in each $C\equiv N$ triple bond). ($3\sigma, 4\pi$) .
   - **$CH_2(CN)_2$ (Malononitrile):** The structure is $NC-CH_2-CN$. It contains 6 $\sigma$ bonds (two C-H, two C-C, two C-N) and 4 $\pi$ bonds (two in each $C\equiv N$). ($6\sigma, 4\pi$)

3. **Conclusion:** Only $XeO_4$ has an equal number of sigma and pi bonds.

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