Towards the end of the 18th century, two different laws were put forward to explain chemical reactions. The first law was the law of definite proportions, which stated that all chemical compounds feature their component elements in fixed ratios by mass. The second law was the law of conservation of mass (also known as the principle of mass conservation), which stated that for any closed system involving mass and energy transfers, the mass of the system must remain constant. In other words, mass can neither be created nor destroyed.

 

The law of conservation of mass is believed to have been developed by the French chemist Antoine Lavoisier. The law of definite proportions was put forward by the French chemist Joseph Proust. It can also be noted that the law of definite proportions is also called Proust’s law or the law of constant proportions. The English chemist John Dalton studied both these theories and expanded the concept by putting forward his atomic theory, commonly referred to as Dalton’s atomic theory. Dalton theorized that matter was made up of small, indivisible units that he called ‘atoms’.

What were the Postulates of Dalton’s Atomic Theory?

The key postulates of Dalton’s atomic theory are listed below.

  • Postulate 1 – All matter is made up of atoms.
  • Postulate 2 – All atoms of a given element have exactly the same masses and properties (they are identical in mass and properties)
  • Postulate 3 – Compounds are formed from the combinations of two or more atoms.
  • Postulate 4 – A chemical reaction involves the rearrangement of atoms between the reacting substances.

Through these postulates, John Dalton suggested that chemical reactions feature hundreds of millions of atoms that react with each other to form molecules. For example, sodium chloride (common salt) is formed when sodium atoms react with chlorine atoms.

What are the Merits of Dalton’s Atomic Theory?

  • Dalton’s atomic theory was the first theory to attempt to describe matter in terms of atoms and explain its properties.
  • Dalton’s atomic theory was based on, and complied with, the law of conservation of mass and the law of constant proportions (also known as the law of definite proportions).
  • Dalton correctly suggested that all matter can be broken down into atoms. He was, however, mistaken about the indivisibility of atoms.
  • Dalton also correctly suggested that all compounds are made up of molecules that are formed from combinations of different atoms. Follow the hyperlink to access an article on what is a molecule. Dalton also correctly suggested that chemical reactions feature the rearrangement of atoms.

What are the Shortcomings of Dalton’s Atomic Theory?

It incorrectly suggested that atoms are indivisible

Atoms can be broken down into several subatomic particles such as protons, electrons, and neutrons.

Dalton’s atomic theory did not account for the existence of isotopes

Dalton’s atomic theory stated that all atoms of a given element have identical masses and identical properties. However, different isotopes of the same element can have different atomic masses. This was not accounted for by Dalton’s atomic theory.