DALTON’S ATOMIC THEORY
On the basis of laws of chemical combination John Dalton, an English school teacher in Manchester, proposed that behavior of matter could be explained using an atomic theory.
He published his work about atomic theory in 1808.
The main points of Dalton’s atomic theory are:
1. Matter is composed of extremely small indivisible particles called atoms.
2. An element consists of only one type of atoms. A sample of an element that is large enough to see, contains a very large number of atoms, but all the atoms of a particular element have identical properties such as mass, shape, colour, density, chemical properties, etc.
3. Atoms of one element differ from atoms of all other elements in mass and chemical properties.
4. Atom is the smallest particle that takes part in chemical reactions.
5. Atoms of different elements may combine with each other in a fixed, simple whole number ratio to form compound atoms or molecules.
6. Atoms can neither be created nor destroyed during chemical reactions.
Dalton suggested symbols for the atoms of different elements as shown in Fig. 5.1.
Fig. 5.1. Symbols for some elements as proposed by Dalton.
He was the first scientist to use the symbols for elements in a quantitative sense. When he used symbol for an element he meant a definite quantity of that element, that is, one atom of the element.
LIMITATIONS OF DALTON’S ATOMIC THEORY
Dalton’s atomic theory was the first successful attempt which gave us some idea about the inner structure of matter. It could very well explain the laws of chemical combination by mass. The main failures of Dalton’s atomic theory are:
1. It failed to explain how atoms of different elements differ from each other, i.e., it did not tell anything about internal structure of the atom.
2. It could not explain how and why atoms of different elements combine with each other to form compound atoms or molecules.
Dalton’s atomic theory held the ground for about a century and no changes were made in it. However, towards the end of 19th century and in the beginning of 20th century new discoveries were made by Sir J.J. Thomson, Neils Bohr, Chadwick and others which revealed the inner structure of atom. In the light of these findings Dalton’s atomic theory was suitably modified.