Dalton’s Atomic Theory: Postulates, Limitations & Drawbacks

Ever wondered what the world is made up of? If we were to zoom in \(~100000000000\) times the skin of our fingertip, what would we see? Would that look any different from zooming in on a banana? On cutting the banana into pieces, would we reach a point where the pieces could no longer be cut any smaller? What would those pieces look like? Would they still have the properties of bananas?

The answers to these questions weren’t available until a few hundred years ago. Thanks to scientists such as John Dalton. The world today thinks in terms of atoms because of John Dalton. Dalton’s atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. Though the atoms are invisible to our naked eye, properties of matter such as colour, phase (e.g., solid, liquid, gas), and even smell come from interactions on an atomic level.

Who was John Dalton?

John Dalton \((1766-1844)\) was an English chemist, physicist, and meteorologist. He was best known for introducing the atomic theory into chemistry and for his work on human optics. He had an unusual perception of colour, which inspired him to conduct the first-ever research into colour blindness – a subject which subsequently became known as Daltonism.

Due to his intense curious nature, he joined his older brother in running a local school at the age of \(12\). There he remained as a teacher for over a decade.

Dalton had a keen interest in studying weather conditions. He thoroughly studied the absorption of water vapour by air at different temperatures. His subsequent work on the constitution, evaporation and thermal expansion of gases led to his derivation of Dalton’s law of partial pressures. This in turn led to the formulation of the atomic theory. While some of the assumptions were incorrect, his ideas were revolutionary and laid the foundations for the development of all modern chemistry.

Basis for Dalton’s Theory

A. The notion of the existence of atoms goes far back two millennia, even before \(1803\) when John Dalton gave us the idea of atoms.

B. An ancient Indian philosopher named Kanada talked about the existence of indivisible particles, which he called “anu”.

C. In \(1773\), French chemist Lavoisier proposed the law of conservation of matter. In which he proposed that matter is always conserved in a chemical reaction. It is neither created nor destroyed in a chemical reaction. This means that in a chemical reaction, the amount of each element in the reactants is equal to that of the products formed. We use the law of conservation of mass every time we balance chemical equations!

D. Later, Joseph Proust, a French chemist, proposed his law of definite proportions. This law stated that a given chemical compound always contains its component elements in a fixed ratio (by mass) and does not depend on its source and method of preparation. This law was also known as Proust’s law.

E. For example, table salt, with the molecular formula \({\rm{NaCl,}}\) has sodium and chloride ions in the ratio \(1:1\). It contains the same proportions of the elements sodium and chlorine irrespective of the amount and source from where it is taken.

F. A few years later, in \(1803\), John Dalton came up with his law of multiple proportions. This law states that if two elements form more than one compound, then the ratios of the masses of the second element, which combine with a fixed mass of the first element, will always be ratios of small whole numbers.

G. To support all his previous works and the law of multiple proportions, Dalton formulated the atomic theory. He published his work in the book A New Chemical Philosophy.

H. Dalton believed that his atomic theory could explain the absorption of gases by water in different proportions: for example, he found out that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesised that this was due to the differences in the mass and complexity of the individual particles that constitute the gases.

Postulates of Dalton’s Atomic Theory

The postulates of Dalton’s atomic theory, which he materialised in \(1808\), are discussed below:

A. All substances are made up of matter. Matter consists of small particles called atoms. Atoms are fundamental particles that are indestructible and indivisible.
Dalton hypothesised that the law of definite proportions and the law of conservation of mass could be explained using the idea of atoms. He proposed that matter consists of tiny indivisible particles called atoms. He imagined the atom to be “tiny, solid, hard, massy and impenetrable, movable particle(s)”. Dalton did not have any insight into the internal structure of the atoms. We might visualise Dalton’s atoms as solid spheres in a molecular modelling kit.
B. For a given element, all its atoms are identical in mass, size, and other physical and chemical properties.
Dalton hypothesised that every single atom of a particular element, such as gold, is the same as every other atom of that element. He also noted that no two elements have the same set of properties. The atoms of a particular element differed from the atoms of other elements. This holds even today. A calcium atom is different from an oxygen atom. There may exist a similarity in boiling points, melting points, and electronegativities, but they differ greatly in their constituting atoms.
C. Different elements have different atoms, which differ from each other in mass and chemical properties.
D. 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.
Dalton proposed that compounds are formed by the combination of two or more different kinds of atoms. For example-water. Two atoms of oxygen combine with one atom of oxygen to form a compound of water i.e. \(\mathrm{H}_{2} \mathrm{O}\). Both hydrogen and oxygen are gases, but their resultant compound is a liquid. It would not make sense to write the formula of water as \(\mathrm{H}_{1.5} \mathrm{O}\) since atoms are indivisible, they will always combine in simple whole-number ratios only. We can’t have half of an atom!

E. Atoms are neither destroyed nor created; This is also true in a chemical reaction.

In a chemical reaction, the properties of the combining atoms are always retained. The number of atoms present on the reactant side is the same as the number of atoms formed on the product side. The mass of the combined atoms remains fixed.

F. A chemical reaction is a rearrangement of atoms.
Dalton proposed that in a chemical reaction, atoms are neither created nor destroyed. Rather they rearrange among themselves. When sodium combines with chlorine to make salt, both the sodium and chlorine atoms still exist. They simply rearrange among themselves to form a new compound that has properties way different from that of the individual atoms. In a chemical reaction transformation of atoms of an element to atoms of another element does not take place.

Based on his atomic theory, Dalton assigned various symbols to elements in circular forms, most of which do not exist today.

Drawback or Limitations of Dalton’s Atomic Theory

Though Dalton’s atomic theory gave an idea about the existence of atoms, it had many shortcomings, which are discussed below:

A. Dalton stated atoms are fundamental, indivisible particles. This statement is false because we know atoms consist of electrons and nucleons (protons and neutrons). It does not account for the presence of subatomic particles.

B. Dalton’s proposition of the atom being indestructible did not hold. Nuclear fission and fusion reactions show that atoms can be created and destroyed as well.

C. According to the theory, atoms of a given element are identical in mass. These postulates became invalid with the discovery of isotopes. Isotopes are the atoms of the same element which differ in mass—for example, hydrogen, deuterium, and tritium.

D. Dalton’s theory does not account for isobars. The theory states that atoms of different elements are completely different from each other. But this is proven wrong when elements like calcium and argon share the same atomic mass \({\rm{40\;amu}}{\rm{.}}\)

E. As per the theory, atoms should always combine in simple whole-number ratios to form a compound. But this postulate is invalid because elements need not combine in simple ratios only. Certain non-stoichiometric compounds and many heavyweight molecules complex organic compounds like sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\).

F. The theory does not account for the existence of allotropes. It fails to differentiate in the properties of charcoal, graphite, and diamond, all of which contain carbon only.

Significance of Dalton’s Atomic Theory

Dalton’s atomic theory gave the first understanding of the atom. Before it was just an abstract philosophical concept. The structure of the atom that we know today is the essence of Dalton’s atomic model.

Merits of Dalton’s Atomic Theory

A. Dalton’s theory stands true for several fundamental laws of chemical combination such as the law of definite proportions, the law of multiple proportions, and the law of conservation of mass.
B. The theory provides a basis to differentiate between elements and compounds

Summary

Despite the shortcomings, Dalton’s atomic theory forms the framework of modern chemistry. This theory has enabled scientists to see the world on an atomic level. Although its two centuries old, Dalton’s atomic theory remains valid in modern chemical thought.

FAQs on Dalton’s Atomic Theory

Q. Mention any three postulates of Dalton’s Atomic Theory.
Ans: The three postulates of Dalton’s Atomic Theory are –
a. All matter consists of tiny indestructible particles called atoms.
b. All atoms of a particular element are identical in size, mass, and other properties.
c. Atoms are neither created nor destroyed in a chemical reaction.

Q. Which atomic model is used today?
Ans: Bohr‘s atomic model is used today. Bohr’s atomic model depicts a central atomic nucleus and oval lines reflecting the electron orbits.

Q. What are the shortcomings of Dalton’s Atomic Theory?
Ans: Some important demerits of Dalton’s atomic theory are listed below.

a. The theory stood invalid in explaining the indivisibility nature of atoms. Atom can be subdivided into protons and nucleons.
b. The theory suggested that all atoms of an element must have identical masses and sizes, which is not the case with isotopes.
c. This theory also did not account for the existence of isobars (nuclides of different chemical elements with the same mass number).
d. Dalton’s atomic theory failed to account for the existence of allotropes.
e. This theory also stated that elements must combine in simple, whole-number ratios to form compounds. However, this did not hold for complex organic compounds like glucose.

Q. How can we differentiate between elements and compounds based on Dalton’s Atomic Theory?
Ans: Dalton’s theory states that elements combine in fixed, whole-number ratios to form compounds. Therefore, it suggests that compounds are made up of molecules that contain two or more atoms of different elements.