All matter consists of indivisible particles called atoms.Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.Atoms cannot be created or destroyed.Atoms of different elements may combine with each other in a fixed, simple, whole number ratios to form compound atoms.Atoms of same element can combine in more than one ratio to form two or more compounds.The atomĀ is the smallest unit of matter that can take part in a chemical reaction.