 In this video we're going to clarify the difference between molecular and lattice structures. We've now looked at covalent bonding, ionic bonding and metallic bonding, which covers all the chemical bonding possible. And you may recall that the primary thing that determines what kind of bonding will occur between two atoms is their electronegativity. We can now classify chemical substances according to whether their bonding creates separate individual molecules or if it causes a three-dimensional network which we call a lattice. If a substance comes as molecules, it means its bonding must be covalent. This kind of bonding where the electrons are localized in a bond between the two atoms is the only kind that gives discrete molecules. Water is an example of a molecular substance. Its molecules consist of exactly one oxygen atom covalently bonded to two hydrogen atoms, no more and no less. So the formula of a molecular compound, H2O in this instance, tells you the exact number of atoms in one molecule of that substance. The other possibility is that the substance forms a lattice and there are three ways that this can occur. The substance could be ionic and the lattice is formed by a regular alternate arrangement of cations and anions. Or it could be metallic with a lattice of metal ions surrounded by a sea of electrons. This could be for a pure metal or it could be for an alloy. And the last possibility is that the bonding is covalent. But the structure is such that it can be infinitely extended in all directions as long as you don't run out of atoms to bond. This type of compound is called a covalent network solid.