Question

Which one of the following species is not likely to exist? $H_{2}^{+}$, HeH, $He_{2}$.

Answer 1

$\sf{He_2}$ doesn't exist. This is because He ( Helium ) is a noble gas and it's octet is fully complete. So, He doesn't react with anyone.

Answer 2

In the hydrogen atom $H_{2}^{+}$, the bond order is 0.5. Since $H_{2}^{+}$ has a positive bond order, so it is stable and exists.

In the HeH species too, the bond order is positive. Since HeH has a positive bond order and so it is stable. HeH also exists.  

In the Helium atom $He_{2}$, the bond order is zero. Since $He_{2}$ has a zero bond order and hence it is not stable. So, Helium atom $He_{2}$ not likely to exist.