Dalton atomic Theory Introduction

John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:

• All substances are made up of tiny, indivisible particles called atoms.

• Atoms of the same element are identical in shape, size, mass and other properties.

• Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.

• Atom is the smallest unit that takes part in chemical combinations.

• Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.

• Atoms cannot be created, divided or destroyed during any chemical or physical change.

The main achievement of Dalton's theory was the derivation of the laws of chemical combination. But later discoveries found that it could not explain:

• the law of gaseous volumes

• why atoms of different elements have different masses, sizes, valencies etc.

• why atoms of different elements combine with each other to form molecules.

• the nature of forces that bind together atoms in a molecule

• the fundamental particles that make the element or the compound