John Dalton’s Atomic Theory – Postulates & Limitations

Dalton’s Atomic Theory

John Dalton, who had one of the finest minds of his time, was born to a poor weaver and started his career as a teacher in a village school. Later he taught mathematics, chemistry, and physics at a college in Manchester. His passion, however, was studying physical and chemical phenomena. On the basis of his own studies and the laws of chemical combination he came to the conclusion that elements are made of atoms. This was a great leap forward for science and the theory put forward by him is called Dalton’s atomic theory in his honor. The main postulates of his theory, which was published in 1808, are given below.

1. Matter is made up of small, individual particles called atoms which are indivisible.
2. The atoms of a particular element are identical in all respects (i.e., shape, size, mass and chemical properties) but differ from atoms of other elements.
3. An atom is the smallest unit that takes part in chemical combinations.
4. Atoms of two or more elements combine in a simple, whole-number ratio to form compound atoms (now called molecules).
5. Atoms can neither be created nor destroyed in a chemical reaction.

At the time when Dalton put forward his theory, most scientists were already of the view that matter consists of atoms. What Dalton did was to make this idea more concrete by assigning specific properties to the atoms of elements. In other words, he said atoms of a particular element have particular properties like shape, size, and most importantly, mass. He reasoned that only this concept of atoms could explain the laws of chemical combination.

• He said, for example, that the atoms of a particular element are identical but differ from atoms of other elements. This explains why different samples of the same element behave identically (have the same properties) but samples of different elements behave differently.
• According to Dalton, atoms are indestructible. This explains the law of conservation of mass. If atoms cannot be created or destroyed then the mass of the reactants must be the same as the mass of the products in a chemical reaction.
  That an atom of an element has a fixed mass can explain why a fixed quantity of an element (containing a particular number of atpm§) has a fixed mass.

Dalton also said that atoms combine (in simple ratios) to form compound atoms molecules were called compound atoms then). This explains both the law of definite proportions and the law of multiple proportions. Dalton actually proposed his theory on the basis of the laws of conservation of mass and definite proportions. It was he who put forward the law pf multiple proportions as a logical consequence of his theory.

• Dalton’s theory was a great milestone and provided an impetus to other scientists to investigate the structure of matter, but it had certain shortcomings. He did say that an atom has a definite mass but it was not possible at that time to determine this mass. Nor was it possible to determine the formulae of compounds (which would indicate how many atoms pf different elements combined to form a molecule of a particular compound).
• One had to start somewhere, and Dalton started by assuming that all elements are monoatomic and that these individual atoms combine in the simplest possible ways to form molecules. Accordingly, water and ammonia were assigned the formulae OH and NH respectively. Now we know that this is not true.
• Dalton’s theory could also not explain Gay-Lussac’s law, though it could explain the laws of chemical combination related to the masses of reactants and products. Nothing was known at that time about the relationship between the volume of a gas and the number of atoms it contains. Subsequent work by several scientists like Thomson, Rutherford and Niels Bohr has shown that Dalton’s theory had the following drawbacks.

1. Dalton did say that the atoms of different elements are different but he could not explain the reason. This came to be known only later when the constituents of the atom (protons, neutrons and electrons) were discovered.
2. Dalton could only say that atoms combine to form compound atoms (molecules); his theory said nothing about the nature of atoms or the kind of molecules they may form. Molecules were just thought of as a combination of atoms.
3. Dalton’s theory could not explain why and how atoms combine to form compound atoms (molecules).
4. Since nothing was known about the structure of an atom, his theory did not say anything about the nature of the forces that hold atoms together in a compound atom (molecule).
5. Dalton could explain the laws of chemical combination related to masses of reactants and products but he could not explain the law of gaseous volumes.

Dalton’s atomic theory has been modified in view of the picture we now have of the structure of the atom. This picture evolved over years, thanks to the painstaking work of several scientists and some brilliant flashes of insight. For the moment the salient features of the modem atomic theory have been listed below.

1. An atom is the smallest particle of matter which takes part in a chemical reaction.
2. An atom has the properties of the element from which it is derived.
3. An atom is composed of electrons, protons, and neutrons, it is no longer considered to be indivisible.
4. Atoms of the same element may have different atomic masses, that is to say, atoms of the same element need not be identical in every respect. When atoms of the same element have different atomic masses they are called isotopes. Isotopes have the same chemical properties.
5. Atoms of different elements may have the same atomic mass, for example, calcium and argon (40 amu). Such atoms are called isobars. This means atoms of different elements can be similar in some respects.
6. Now we know that atoms of one element can be changed into atoms of other elements (in nuclear reactors, for example). We can no longer think that they are indestructible since such changes always involve the conversion of some mass into energy. Such changes are called transmutation. However, in the course of chemical reactions, atoms remain unchanged, and we can think of them as indestructible.
7. Atoms do not always combine in simple ratios to form compounds, e.g., in proteins or carbohydrates, the ratios in which the elements combine are not simple, though they are fixed. For example, in a sugar molecule (C₁₂H₂₂C₁₁) the ratio of C, H, and O atoms is not simple, it is 12:22:11.