The bonds that unite the different atoms that form a chemical compound can be ionic or covalent. In covalent bonds, electrons are shared, while ionic bonds involve the transfer of electrons between atoms. Ionic bonds usually form between metals and non-metals (NaCl). Covalent bonds are frequent between non-metallic elements.
Molecular compounds, are formed by molecules, consisting of atoms joined by covalent bond. Molecules are represented by a chemical formula that indicates the types of atoms present and the proportion in which they participate.
H 2 O: molecule formed by hydrogen and oxygen in a ratio of 2 to 1.
CCl 4 : molecule formed by carbon and hydrogen in a ratio of 1 to 4
Ionic compounds are formed by combining metals with non-metals. The atoms of metallic elements tend to lose electrons that the atoms of non-metallic elements gain. When a metal loses electrons it forms a positive species called a cation, those electrons are gained by the non-metallic element forming a negative species called anion. Electrostatic forces attract cations and anions to form the ionic compound.
Sodium chloride (common salt) is formed by the transfer of an electron from sodium to chlorine, forming the sodium cation and the chloride anion. Na + and Cl - attract each other to form a crystal whose formula unit is NaCl. We cannot consider NaCl as a molecule since each sodium ion is surrounded by 6 chlorides and vice versa.