Matter is made up of discrete particles which includes atoms, molecules and ions. An atom can be defined as the smallest part of an element that can take part in a chemical reaction. It is the smallest constituent unit of matter that possess the properties of the chemical element.

Some atoms cannot exist alone but in combined state as molecules. A molecule can be defined as a combination of two or more atoms held by a chemical bond . It is the smallest part of a chemical substance that can exist alone and still retain the chemical attributes of that substance.

Atomicity is defined as the number of atoms present in a molecule of an element, compound or substance. Most gases like hydrogen, oxygen and chlorine exists as diatomic molecules. Others like Phosphorus and sulphur exists as polyatomic elements. While noble gases like Helium and Neon exists as monoatomic molecules. Compounds also exists molecules, the atomicity of a compound is the number of atoms in that compound. For example HCl has an atomicity of 2.

An ion can simply be defined as a charged particle. An ion is an atom or molecule that is positively or negatively charged due to the loss or gain of an electron. Cations are positively charged ions while anions are negatively charged ions. When atoms loose electrons, they form cations. For example,
Na - e^- = Na⁺
Al - 3e^- = Al³⁺
Li - e^- = Li⁺
Ca - 2e^- = Ca²⁺
When atoms gain electrons, they form anions which are negatively charged. For example;
O + 2e^- = O^2-
Cl + e^- = Cl^-
N + 3e^- = N^3-