A formula unit is essentially the lowest ratio in which the elements or ions combine to form an extended network of a compound. These are not discrete molecules but a network of ionic or covalent compounds.
In the given examples, AlP is a correct formula unit which is a combination of a metal (Al3+) and a non-metal (P3-).
In the remaining examples:
PbCl is incorrect as it exists as PbCl2
Li3Al is a combination of 2 metals
ClO can exist as independent/isolated molecules and not as a network
There are two types of substances mixture and pure substance. Mixture has NO chemical formula and a pure substance has a chemical formula. There are two types of pure substances, elements (mono atomic and molecular) and compounds ( covalent and ionic).
Ionic compounds do not exist in independent molecular form. They form three dimensional crystal lattice, in which each ion is surrounded by oppositely charged ion. so the ratio of ion is called the formula unit
