So we have two electrons in our bonding orbital, but because we use the same rules to fill up molecular orbitals as we do atomic orbitals, so the Pauli exclusion principle tells us we can't have more than two electrons per orbital, so we have to go up to our anti-bonding orbital here.

所以在成键轨道上有两个电子，但因为我们用了和原子轨道时,用的相同的规则，所以Pauli不相容原理告诉我们,一个轨道上不能有两个以上的电子，所以我们需要填充到反键轨道上去。

麻省理工公开课 - 化学原理课程节选