According to law of constant proportions, a sample of a pure substance always consists of the same elements combined in the same proportion by mass.

When an element combines with another element and forms more than one compound, then different masses of the one element that combine with the fixed mass of another element are in the ratio of simple whole number or integer. This is the law of multiple proportions.

A molecule is the smallest particle of an element or of a compound which shows all properties of that substance and can exist freely under ordinary conditions.

Atom of the isotope C-12 is assigned atomic mass unit of 12 and the relative atomic masses of all other atoms of elements are obtained by comparing them with it. The mole is the amount of a substance which contains the same number of particles (atoms, ions or molecule) as there are atoms in exactly 0.012 kg of carbon-12.

Avogadro’s number is defined as the number of atoms in exactly 0.012 kg (or 12 g) of C-12 and is equal to 6.02 × 1023.

Mass of one mole atoms or one mole molecules or one mole of formula unit of a substance is its molar mass

The composition of any compound can be represented by its formula. For writing the formula of a compound, valency of an element is used. This is normally done in case of covalent compounds. Valency is the combining capacity of an element.

Atomic Number: The number of protons in the nucleus is called atomic number and is denoted by Z. Total number of protons and neutrons is called mass number and is denoted by A.

Isotope: Atoms of an element that have the same atomic number (Z) but different mass number (A).