CBSE Class 9 Answered

Daltons atomic theory.

 Dalton's Atomic Theory - This is a set of fundamental rules about the nature of atoms of which all other rules are based on. It was put forth by John Dalton (1766-1844), an English scientist.
(1) Matter consists of definite particles called atoms.
(2) Atoms are indestructable. They can rearrange in chemical reactions but they do not themselves break apart.
(3) Atoms of a particular element are indistinguishable from one another. They are all identical in mass, as well as other properties.
(4) Atoms of different elements (or types) differ in mass (and other properties).
(5) When atoms of different elements combine to form compounds, new and more complex particles (molecules) form. Their constituent atoms are always present in a definite numerical ratio.

The theory breaks down when going beyond chemical reactions. For example, when large amount of energy is introduced (well beyond the range of chemical energy) atoms will break apart. This occurs in particle colliders or 'atom-smashing' machines.