Molecules
Molecules are structural units that result from the bonding together of two or more atoms. This involves a type of bond known as a covalent bond in which electrons are shared by atoms. The bond consists of the attractive force of the shared electrons between the two nuclei. The atoms are close enough that their atomic orbitals ovelap to form molecular orbitals.
Ionic compounds such as NaCl are ionic as solid, molten liquid or in solution. The sodium atoms have lost the loosely held outer electron, becoming a positive ion, and thereby achieved the electronic structure of the noble gas helium. The chlorine atoms have acquired an electron, becoming the negative chloride ion, and achieving the electronic structure of the noble gas argon. Such ionic compounds do not form molecules. There is no such unit as NaCl. It does not represent a molecule but simply a combining ratio, indicating that there are equal numbers of sodium and chloride ions in sodium chloride. A mole of sodium chloride is thus really a formula weight rather than a true molecular weight, although the term molecular weight is still used for such ionic compounds.
Ionic compounds include atoms that differ considerably in electronegativity. Bonds that form between atoms with similar or identical electronegativity form covalent bonds. These bonds are not easily separated as are ionic bonds. Of course diatomic molecules of the same element, such as H2 or N2, are examples of covalent bonds. Bonds between carbon and hydrogen are covalent, thus compounds such as CH4, C2H6 form molecules. The bond between nitrogen and carbon is covalent, as is the bond between nitrogen and hydrogen as in ammonia NH3.
Some compounds consist of covalent bonds and ionic bonds. The ammonium ion in ammonium chloride contains the ammonia molecule with an added proton. The bond between this and the chloride ion is ionic and the two ions, ammonium and chloride, are independent in solution.