Question

Helium molecule is monoatomic because

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

Answer 1

Answer: (B)

$H{e}_2:\sigma 1s^2,{\sigma }^{\ast }1s^2$
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, $H{e}_2$ does not exist, which is actually so.