Dalton’s Atomic Theory

Today, we know every matter around us is composed of atoms. But this fact was a mystery until the end of the 18^th century when Dalton, an English chemist, proposed his atomic theory. Dalton’s atomic theory was the first scientific atomic theory based on his experiments and examinations of previous scientific works. Modern atomic theory is much different from what Dalton had proposed, but some of the ideas of the theory are still valid. Dalton’s theory provided a foundation for modern chemistry.

History

Although the Dalton stated his atomic theory in 1803, the notion of the existence of atoms goes far back two millennia. Kanada was an ancient Indian philosopher, who talked about the existence of indivisible particles, which he called anu. A similar idea was also conjectured by Democritus, an ancient Greek philosopher. Democritus called these fundamental, indivisible particles as atomos. In 1773, French chemist Lavoisier had discovered the law of conservation of matter. He proposed matter is always conservation in a chemical reaction. This law pushed curious chemists to investigate and better understand chemical reactions. In the same century, Proust, also a French chemist, proposed his law of definite proportions. It was a controversial law at that time because many chemists opposed the law. Few years after Proust’s law, Dalton came with his law of multiple proportions in 1803. To support all previous works and his law of multiple proportions, he formulated the atomic theory. He published his work in the book A New Chemical Philosophy.

Postulates of Dalton’s Atomic Theory

The postulates of Dalton’s atomic theory, which he materialised in 1808, are discussed below.

• Matter consists of small particles called atoms. Atoms are fundamental particles which are indestructible and indivisible.
• For a given element, all its atoms are identical in mass and properties.
• Different elements have different atoms, which differ with each other in mass and chemical properties.
• Compounds are formed from the combination of two or more atoms of different elements. These atoms always combine in simple ratios to give a compound.
• Atoms are neither destroyed or created; This is also true in a chemical reaction.
• A chemical reaction is a rearrangement of atoms.

Dalton’s Work

Dalton was not only a chemist but also a physicist and meteorologist. He is famous for his atomic theory and law of partial pressure and, his research on colour blindness. Dalton had a deep interest in meteorology. He daily maintained records of the weather for fifty-seven years. It was his intense interest in the atmosphere led him to study gases and consequently, resulted in the atomic theory.

While studying gases, Dalton discovered elements always combines in a fixed proportion based on mass or volume to form a compound. He observed that a compound always contains the same proportion of elements by mass. This provided support to Proust’s law of definite proportions. Dalton also observed that given elements can combine in different proportions to form more than one compound, for example, carbon and oxygen can form two different compounds. His further examination resulted in the law of multiple proportions.

To support his experimental findings, he created the first scientific atomic theory. The postulates of the theory are already mentioned above. Dalton stated every matter is made up of tiny particles called atoms. He further explained the reason an element is pure is because of all atoms in an element are identical in all respect. Dalton also used the law of conservation of mass to prove that every atom is indestructible. He believed that they all are spherical. According to him, a chemical reaction is a rearrangement of atoms. No atom is created or destroyed in a chemical reaction. There is no transformation of atoms of an element to atoms of another element. Dalton explained a compound as atoms of different elements attached together by some binding.

Consider an example of water. From Lavoisier’s work, Dalton knew the water was 85 % oxygen and 25 % hydrogen by mass. So, Dalton assumed that water contains one atom and one atom of hydrogen such that the mass of oxygen is 5.67 times the mass of hydrogen. The figure below illustrates the same.

As we can see from the above figure Dalton’s water molecule contains 1 hydrogen and 1 oxygen atom, which is not true. A molecule of water contains 2-hydrogen and 1 oxygen atom. The molecular formula of water is H₂O, not HO.

Base on the above methodology, Dalton tabulated relative atomic weights of some elements and gave symbols to various elements. The figures below illustrate the same.

As we can see from both the figures (above and below), Dalton assigned various symbols to elements in circular forms. These symbols are no longer valid today. Also, the structures of atoms and molecules are far more different than Dalton’s.

Limitations or Drawbacks of Dalton’s Atomic Theory

Dalton’s atomic theory is not a perfect atomic theory. In fact, it has many drawbacks, which are covered below.

• Dalton stated atoms are fundamental particles which are indivisible. This statement is false because we know atoms consist of electrons and nucleons (protons and neutrons).
• The theory states the atoms are indestructible. This is no longer true. We, today, know that atoms are destroyed or created in nuclear fission and fusion respectively.
• According to the theory, atoms of a given element are identical in mass. With the discovery of isotopes, this point of postulates became invalid. Isotopes are atoms of the same element which differ in mass. For example, two major isotopes of uranium are ²³⁵
  ₉₂U and ²³⁸
  ₉₂U.
• Dalton’s theory claims atoms of different elements are completely different. This is proven wrong in some cases. For example, argon and calcium are different elements, but they have the same atomic mass (40 amu).
• As per the theory, atoms always combine in simple ratios to form a compound. This is not true in the case of non-stoichiometric compounds and many heavyweight molecules.
• The theory does not account for the existence of allotropes. It fails to differentiate among charcoal, graphite, and diamond.

Merits of Dalton’s Atomic Theory

Although there are many drawbacks of Dalton’s atomic theory, it played a crucial role in the development of modern atomic theory. Dalton’s atomic theory was able to explain many of the unanswered questions in the 19^th century. The claims of the theory which are still valid are mentioned below.

• The theory can explain the law of definite proportions and the law of multiple proportions.
• Dalton’s was the first to explain the difference between compounds and elements.

Associated Articles

• Law of Conservation
• Law of Definite Proportions
• Law of Multiple Proportions
• Law of Reciprocal Proportions
• Dalton’s Law of Partial Pressure