- A molecule can be expressed in form of a chemical formula using symbols of constituent elements.

- A molecular formula shows the actual number of atoms of different elements in a molecule of an element or of compound. In other words, composition of any compound can be represented by its formula. For writing formula of a compound valence or valency of the elements is used. Valency is combining capacity of an element and is related to its electronic configuration.

- An empirical formula shows the simplest whole number ratio of the atoms of different elements present in a compound.

- Atoms of the isotope ¹²C are assigned a atomic mass unit of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12.

- The mole is the amount of substance which contains the same number of particles (atoms, ions or molecules) as there are atoms in exactly 0.012 kg of ¹²C.

- Avogadros constant is defined as the number of atoms in exactly 12 g of C-12 and is equal to 6.022 × 10²³ mol-1.

- Mass of one mole atoms or one mole molecules of a substance is its molar mass and volume of one mole of the substance is its molar volume.

- A chemical equation is a shorthand description of a reaction. A balance chemical equation provides quantitative information about reactants consumed and products formed in a chemical reaction. A balance chemical equation obeys law of conservation of mass and law of constant proportions.