Dalton’s Atomic Theory: The word ‘atom’ was derived from Greek word ‘atomos’ which means indivisible. The philosopher Democritus created an idea about the atom. According to him, every mater is composed of tiny particles called an atom. This idea was not properly used and explained until John Dalton(1766-1844). John Dalton published a series of articles called ‘A New System of Chemical Philosophy’ in which he explained about laws of combination of chemicals during a reaction.

The main postulates of Dalton Atomic theory are:

1. Ever matter is composed of extremely small indivisible discrete particles known as atoms and atoms can neither be created nor destroyed by any chemical means or reaction
2. All the atoms of same element are identical in all the respects.
3. The atoms of different element have different mass numbers and properties.
4. Two or more than two atoms combine in simple whole-number ratio during compound formation.
5. The compound atoms of the same compound are identical in all respects.

LIMITATIONS:

1. After the discovery of radioactivity, it was found that one type of atom can be transformed into another type of atom by fission or fusion.
2. The isotopes of an element have different mass number. So, the properties of isotopes are somehow different. For eg: protium, deuterium, and H (Tritium)are the isotopes of hydrogen. Protium is more stable than others since it has less mass atomic number and Tritium is radioactive since it has less mass number among the isotopes of hydrogen.
3. The elements(atoms) which have same mass number but different atomic number are called isobar. eg: Ar and C are isobars having same atomic mass 40 but shows different properties.
4. Large number of compounds are available in which atoms do not combine in simple whole-number ratio. For eg: sugarcane(C₆H₁₂O₆=1.09:2:1) and  Sodium Borax( Na₂B₄O₇= 1:2:3.5)
5. Nowadays, compound atoms are termed as molecules. Molecules of the same compound are not identical due to the presence of isotopes.