Suggest why the noble gases such as neon and argon do not exist as molecules.

Electrons and Stability

The stability of an atom is related to the number and arrangement of its electrons. Atoms with completely filled valence shells tend to be more stable. The valence shell of an atom is its outermost shell. Atoms form compounds such as molecules or ionic salts in order to fill their valence shells with electrons. 

Noble Gases

Noble gas atoms are located in column 18 of the periodic table. The valence shells of these atoms are already completely filled. Therefore, they are already stable and do not need to form compounds in order to fill their valence shells.

Helium and neon never form molecules because their shells are completely filled and they have very high ionization energies. Ionization energy is the amount of energy needed to remove an electron from an atom. The high ionization energies of helium and argon prevent these atoms from forming ions. Some of the other heavier noble gases, such as krypton and argon, will occasionally form molecules.