Matter
Matter is anything that has mass and can occupy space, it consists of small particles. These particles may be atoms, molecules and ions.
ATOM
An atom is the smallest particle of an element that can take part in a chemical reaction. Examples are helium atom, chlorine atom.
MOLECULES
It is the chemical combination of two or more atoms that exists as a separate particle. An atom that forms a molecule may be of the same element or different elements. A molecule is said to be monoatomic when it contains only one atom. A molecule is said to be diatomic when it contains two atoms. A molecule is said to be triatomic when it contains three atoms.
Examples of molecules
ION
This is a charged atom or a group of atoms that exist together as a charged unit. An ion is also an atom or a molecule in which the total number of electrons is not equal to the total number of protons. Examples are; Cl- (Chlorine ion), K+ (Potassium)