Diatomic molecules are those that are capable of forming a bond with themselves.
There are currently 7 diatomic elements, they are:
Hydrogen → H
Oxygen → O
Nitrogen→ N
Fluorine → F
Chlorine → Cl
Bromine → Br
Iodine → I
In this order of ideas, the element that is not capable of forming a bond with itself is Neon, this is a practically inert noble gas, since it has its two complete energy levels.
So, the answer will be Neon
