During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the Law of Conservation of Mass.
In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the Law of Definite Proportions.
An atom is the smallest particle of the element that cannot usually exist independently and retain all its chemical properties. A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.
A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element. The chemical formula of a molecular compound is determined by the valency of each element.
Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them. In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.
Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.
The Avogadro constant, 6.022 x 1023 is defined as the number of atoms in exactly 12 g of carbon-12. The mole is the amount of substance that contains the same number of particles (atoms / ions / molecules / formula units) as there are atoms in exactly 12 g of carbon-12.
Mass of 1 mole of a substance is called its molar mass.