Dalton's Atomic Theory (1808)

The word "atom" comes from the Greek word "atomos" which means indivisible (cannot be divided). A Greek philosopher Democritus (460–370 BC) first suggested this idea.

John Dalton, an English school teacher, presented his atomic theory in 1808 under the title "A New System of Chemical Philosophy."

Main Postulates:

1. All matter is made of very small particles called atoms.
2. An atom is indivisible — it cannot be broken into smaller parts.
3. Atoms can neither be created nor destroyed.
4. All atoms of the same element are identical in size, shape, and mass.
5. Atoms of different elements are different in their properties.
6. Atoms combine with each other in small whole number ratios.
7. All chemical reactions happen because of combination or separation of atoms.

Note: Dalton's model showed the atom as a solid ball (like a cricket ball).