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Q.
If Hund's rule does not hold good, then which of the following pairs is diamagnetic?
Chemical Bonding and Molecular Structure
Solution:
$C _{2}(12):$ - Violating Hund's rule
$\sigma 1 s ^{2}, \sigma^{*} 1 s ^{2}, \sigma 2 s ^{2}, \sigma^{*} 2 s ^{2}, \pi 2 p _{ x }^{2}=\pi 2 p _{ y }^{2}, \sigma 2 p _{ x } \pi^{*} 2 p _{ x }$
$=\pi^{*} 2 p_{y}, \sigma^{*} 2 p_{z}$
$C _{2}$ is diamagnetic.
For $O _{2}(16 e .)$ violating Hund's rule
$\sigma 1 s ^{2}, \sigma^{*} 1 s ^{2}, \sigma 2 s ^{2}, \sigma^{*} 2 s ^{2}, \sigma 2 p _{z}^{2}, \pi 2 p _{ x }^{2}=\pi 2 p _{y}^{2}, \pi^{*} 2 p _{ x }^{2}$