Dalton proposed in his hypothesis that atoms react to form a molecule which he said as "compound atoms". Today we know what a molecule is. A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement. These atoms are held together by chemical forces or chemical bonds.

An atom is the smallest particle of a substance but can not exist freely. Contrary to this, a molecules can be considered as the smallest particle of an element or of a compound which can exist alone or freely under ordinary conditions. A molecule of a substance shows all chemical properties of that substance.

Oxygen molecule is made of two atoms of oxygen and therefore it is a diatomic molecule (represented by O2), hydrogen, nitrogen, fluorine, chlorine, bromine and iodine are other examples of diatomic molecules and are represented as H2, N2, F2, Cl2 , Br2 and I2 respectively.

Some other elements exist as more complex molecules. Phosphorus molecule consists of four atoms (denoted by P4) whereas sulphur exists as eight atom molecule (S8) at ordinary temperature and pressure. A molecule made of four atoms is tetratomic molecule. Normally, molecules consisting of more than three or four atoms are considered under the category of polyatomic molecules.

Molecules of compounds are composed of more than one kind of atoms. For example, water molecule which is composed of more than one kind of atoms. In one water molecule, there are two atoms of hydrogen and one atom of oxygen. It is represented as H2O. A molecule of ammonia consists of one nitrogen atom and three hydrogen atoms. A molecule of ethyl alcohol (C2H5OH) is composed of nine atoms (2 atoms of carbon, 6 atoms of hydrogen and one atom of oxygen).

Molecular Mass

A molecule can be represented in the form of a formula popularly known as molecular formula. Molecular formula may be of an element or of a compound. Molecular formula of a compound is normally used for determining the molecular mass of that substance. If a substance is composed of molecule (for example: CO2, H2O or NH3), it is easy to calculate the molecular mass.

Molecular mass is the sum of atomic masses of all the atoms present in that molecule. For example, the molecular mass of CO2 is obtained as:

C: 1 × 12.0 u = 12.0 u
2 O: 2 × 16.0 u = 32.0 u

Mass of CO2 = 44.0 u

Hence, molecular mass of CO2 = 44.0 u

Similarly, molecular mass of NH3 is:

N: 1 × 14.0 u = 14.0 u
3 H: 3 × 1.08 u = 3.24 u

Mass of NH3 = 17.24 u

For substances which are not molecular in nature, we talk of formula mass. For example, sodium chloride (denoted by formula, NaCl) is an ionic substance. For this, we calculate formula mass, similar to molecular mass. In case of sodium chloride, NaCl:

Formula mass = mass of 1 Na atom + mass of 1 Cl atom

= 23 u + 35.5 u = 58.5 u