Explanation:
Atomic number of hydrogen is 1 and its electronic configuration is [tex]1s^{1}[/tex]. So, in order to attain stability it needs to gain one more electron from another atom.
As helium cannot donate its one valence electron because if it does so then there will be no electron present in a hydrogen atom which is not possible. Therefore, it needs to share its valence electron with another atom.
A chemical bond formed by sharing of electrons is known as a covalent bond. So, hydrogen atom always shares its valence electron and hence, it always forms a covalent bond.
On the other hand, atomic number of helium is 2 and its electronic configuration is [tex]1s^{2}[/tex].
Since, helium atom has completely fill orbital so, it is stable in nature. Therefore, it will neither gain or lose electrons under normal conditions.
As a result, helium will not form a covalent bond.
Helium has a complete outermost shell while hydrogen do not have a complete outermost shell.
The major aim of chemical reaction is to ensure that atoms attain stability. Stability is attained when atoms have a complete outermost shell. We must recall that in helium as well as well as hydrogen, the outermost shell is the 1s shell. This shell contains only two electrons.
Hydrogen has only one electron in this 1s shell while helium has two electrons in the 1s shell. Hence the outermost shell of helium is complete therefore it does not form covalent bond as hydrogen does. Hydrogen forms covalent bond in which two electrons are shared to complete the 1s shell.
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