Each helium atom has 2 electrons, which is already the maximum no. of electrons that can fit in the first electron shell. When the outermost electron shell is full (2 for the first layer, 8 for others), the atom is stable.
Helium atom itself is already stable, so it doesn't need to combine with another helium atom to form a molecule, hence we call it monatomic.
Monatomic molecule , molecule composed of just one atom, and lacking any covalent bonds. The noble gases (He, Ne, Ar, Kr, Xe, and Rn) are all monoatomic, whereas most other gases are at least diatomic. Approximate Composition of the Atmosphere at the Earth's Surface. Nitrogen.
