Dalton’s Atomic Theory

A scientist, John Dalton, carried out an extensive study on atoms. His ideas on atomic theory are as follows:

1. Matter is made up of small indivisible particles called atoms.

2. Atom can neither be created nor destroyed.

3. Atoms of the same element are alike in every aspect but differ from atoms of all other elements.

4. Atoms of different elements have different masses and different chemical properties.

5. When atoms combine with one another, they do so in a simple ratio.

6. Chemical changes result from the combination or separation of atoms.

Dalton used his theory to explain the Law of Chemical combination but his theory could not explain new discoveries like isotopes, electrolysis, and some aspects of organic chemistry.

Modification of Dalton’s Atomic Theory (Modern Atomic Theory):

1. Matter is composed of small particles that are positively charged, negatively charged, and some electrically neutral.

2. Atoms of the same element are all alike but they may have different masses (such atoms are called isotopes).

3. Atoms of different elements can combine to form compounds. Example: 

2H₂ + O₂ \( \scriptsize \rightarrow \) 2H₂O

4. Two or more atoms of the same element can combine to form a molecule. 

Cl + Cl \( \scriptsize \rightarrow \) Cl₂

5. The molecule of a compound consists of the atoms of the elements that form the molecule e.g. 

H₂ + Cl₂ \( \scriptsize \rightarrow \) 2HCl

Constituents of the Atom

From modern atomic theory, atoms are made up of smaller particles – Protons, Electrons, and Neutrons. Some scientists carried out experiments on the constituents of atoms. The scientists include J.J. Thompson, Robert Millikan, Ernest Rutherford, James Chadwick, Henry Mosely, Niels Bohr, etc