Atomic theory was first proposed by John Dalton, an English physicist and a chemist of the 19th century. He was born in England on September 6, 1766. 

His work, Meteorological Observation, and Essays contained many original ideas of his and his friends and was published in 1793. This book created a significant shift in meteorology from folklore to a serious scientific pursuit of the subject. Thus, he was also known as the father of meteorology in some circles. 

He also contributed to Manchester’s literary and philosophical society by publishing a paper on colour blindness called Daltonism for some time. 

He also defined some significant concepts of partial pressure in physics. But his significant contribution to the field of science was the atomic theory based on Dalton’s structure of the atom, also called the Billiard ball model. 

Dalton’s Atomic Theory

The scientific theory that all matter is made up of atoms is the atomic theory. One of the initial theories to be given to explain this was called Dalton’s atomic theory. Dalton was a chemist and an English physicist who put forward his theory on the nature of matter in 1808. Atoms are indivisible, and the structure of atoms is indestructible, and they are the building units forming all the substances. All atoms of a particular element are the same in shape and size, whereas they differ from one element to another. But soon after, the model was discarded due to various demerits of Dalton’s atomic theory.

Postulates Of Dalton’s Atomic Theory: 

• All matter is made up of microscopic, indivisible particles known as atoms.
• A particular element consists of all kinds of atoms with the same mass, size, and other properties. However, different elements exhibit different atoms with different properties and masses and sizes.
• Atoms can neither be created nor be destroyed. They are not further divisible into smaller particles.
• To form compounds, elements of different atoms can combine in fixed ratios that too in whole-number ratio.
• Atoms can be combined, rearranged, or separated in a chemical reaction.

Basis Of Dalton’s Atomic Theory:

Dalton’s theory was based on the following concepts that other scientists coined. These are:

• Law of conservation of mass: This law states that in a chemical reaction, the mass of an element before and after the reaction is the same. That is because mass can neither be created nor be destroyed in a chemical reaction. 

Mass of reactants = Mass of products 

For example- CaCO3 🡪 CaO + CO2

 Molar mass: 100 gm 56 gm 44 gm

By the law of conservation of mass:

100 = 56 +44 

100 = 100

• Law of constant composition:  In a pure compound, the proportion of elements remains constant, i.e. they are always in a fixed ratio concerning their mass. 

For example- NO = 14:16

NO2 = 14:32

N2O = 28:16

Demerits Of Dalton’s Theory

• The theory doesn’t account for subatomic particles

Later after the postulation of Dalton’s atomic theory, subatomic particles were discovered. These were protons, neutrons, and electrons. This proved that atoms were further divisible and were not the basic unit of matter. 

• The theory doesn’t include isotopes.

As one of the postulates states, the masses and densities of an element should be the same. But isotopes of the same element have different atomic masses. Like Isotopes of Hydrogen are deuterium and tritium, their masses are 1,2 and 3, respectively. But atoms are the smallest particle to take part in a chemical reaction.

• The theory doesn’t involve isobars.

According to one postulate, the masses and densities of two different elements must differ. But the discovery of isobars shows that two different elements can have the same atomic masses. Like Isobars of chlorine and argon have 40 atomic masses. 

• Elements are not always present in whole-number ratios in a compound

It is not always necessary that the constituents in a complex compound are present in simple whole-number proportions. For example, Sucrose (C12H22O11) in which C: H: O ratio is 12:22:11, which isn’t a simple whole-number ratio.

• Allotropes are not included in this theory.

Allotropes are substances with the same atomic constituents but different properties like diamond and graphite. Both are made of the same carbon isotope, but their physical properties are opposites. 

Conclusion

The concept of the atom as the building block of the matter was already in the scientific community for a long time. Dalton was the first scientist to scientifically propose the atomic theory, which followed the law of conservation of mass and the law of constant proportion. Dalton’s atomic theory talks about the minor constituent of a matter being the atom and how a particular element is composed of the same type of atoms. With time, newer discoveries were made like the subatomic particles, the isotopes, the isobars, the complex compounds, etc. Dalton’s proposed theory failed to explain these phenomena and became the demerits of Dalton’s atomic theory. Thus, the scientific community discarded this theory, and the significant role played was by Rutherford’s experiment.