The structure of matter has been a subject of speculation from very early times. According to Greek philosopher Democritus, if we go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be divided further. He called these particle as ‘atoms’ meaning indivisible.

All matter is made up of small particles called atoms. Different kinds of atoms and molecules have different properties due to which different kind of matter also show different properties.

Laws of chemical combination : By studying the result of quantitative measurement of many reactions it was observed that whenever substances react, they follow certain laws. These laws are called the law of chemical combination. They are

(a) Law of conservation of mass.

(b) Law of constant proportions.

(c) Law of multiple proportions

Dalton's Atomic Theory: According to this theory, all matter whether element, compound or mixture is composed of small particles called atoms. The theory proposed by Dalton may be stated as follows:

(i) All matter is made up of atoms.

(ii) Atoms are tiny, indivisible and in destructive ultimate particles.

(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole number to form compounds.

(vi) Atoms are neither created nor destroyed.