Complete information on Dalton’s atomic theory, assumptions, merits and demerits are:
It has already been mentioned that the great Indian philosopher Kanad had opined that matter is composed of very small, indivisible particles called parmanus. The ancient Greek philosophers believed that all matter is composed of tiny 'building blocks' which are hard and indivisible. The Greek philosopher Democritus named these building blocks atomos, meaning 'indivisible'. The word 'atom' has been derived from this Greek word.
All these early ideas were mere speculations. These ideas were never put to experimental tests, nor were they ever used to explain scientific truths.
Dalton’s atomic theory
It was John Dalton, a British schoolteacher, who developed a theory of the structure of matter. This theory is now known as Dalton's Atomic Theory. Dalton's atomic theory was a result of the inferences drawn from experiments and the laws of chemical combination.
Assumptions or Postulates of Dalton's Atomic Theory
The main assumptions of Dalton's atomic theory are:
1. All matter is made up of very small particles which cannot be further broken down. These small particles are called atoms.
2. All the atoms of a given element are identical in size, mass, chemical properties etc.
3. Atoms of different elements have different sizes, masses and chemical properties.
4. The atoms do not undergo subdivision during chemical reactions and cannot be created or destroyed by any chemical process, i.e., atoms are indestructible.
5. Chemical combination between two or more elements takes place by the union of the atoms of these elements in simple numerical ratios. For example, 1 atom of A plus 1 atom of B, 2 atoms of A plus 3 atoms of B etc.
6. Atoms of the same elements can combine in more than one ratio to produce more than one compound. For example, carbon and oxygen combine to give carbon monoxide and carbon dioxide. In carbon monoxide, one atom of carbon combines with one atom of oxygen, while mi carbon dioxide, one atom of carbon combines with two atoms of oxygen. Thus, The ratio of carbon (C) and oxygen (O) in carbon monoxide (CO) is 1 :1, while in carbon dioxide, this ratio is 1 : 2.
Merits of Dalton's Atomic Theory
Dalton's atomic theory has explained many things. It has enabled us to explain several observations including the laws of chemical combination.
Demerits of Dalton's Atomic Theory
The following are the main defects of Dalton's atomic theory:
(i) It fails to explain Gay-Lussac's law of gaseous volumes. This law states that the volumes of gases which react, and the volumes of the products, if gaseous, are in the ratio of small natural numbers, if pressure and temperature are kept constant.
(ii) It makes no satisfactory distinction between the ultimate particle of an element and the smallest particle of a compound. Both of them have been called atoms—Dalton called the former simple atoms and the latter compound atoms.
The compound atoms were later called molecules.
(iii) Modern theory refuses to accept that atom is the ultimate indivisible particle of matter.
(iv) The assumption that the atoms of the same element have the same mass, is not true in the case of isotopes of an element.
Note The atoms of the same element which have different mass numbers are called isotopes of the element.
(v) Though charcoal, graphite and diamond are all composed of carbon atoms, their properties are different. This may lead us to believe that all atoms of an element are not identical in all respects. Dalton's atomic theory does not provide any answer to this.
(vi) Simple compounds like carbon dioxide (CO2), water (H2O) etc. could be produced in the laboratory, while compounds like urea, sugar etc. found in living organisms could not be synthesized. This could not be explained by Dalton's atomic theory.