Question

Helium molecule is monoatomic because

• A. no electrons are present in both $\sigma 1s$ and $\sigma^* 1s$ orbitals
• B. two electrons are present in both $\sigma 1s$ and $\sigma^* 1s$ orbitals
• C. $\sigma 1s$ has no electrons and $\sigma^* 1s$ has 2 electrons
• D. $\sigma 1s$ has 2 electrons and $\sigma^* 1s$ has no electrons

Answer 1

Answer: (B)

two electrons are present in both $\sigma 1s$ and $\sigma^* 1s$ orbitals

Answer 2

$He_2 : \sigma 1s^2 , \sigma^* 1s^2$
B.O. $= \frac{1}{2} (N_b - N_a) = \frac{1}{2} ( 2 - 2) = 0$
i.e., no bond is formed between two helium atoms or in other words, $He_2$ does not exist, which is actually so.