John Dalton (1766-1844) was an English physicist, meteorologist, and a well-known chemist of his time. One of his biggest achievements is the introduction of atomic theory. Furthermore, he also worked for color blindness, which is today known as Daltonism, in his honor.
In the year 1808, he introduced a scientific theory on the nature of matter. Although, his atomic theory had some shortcomings afterward, still, it was the first-ever attempt to completely describe matter in terms of atoms. In his book, A New System of Chemical Philosophy, he explained his concepts of heat, specific heat, expansion by heat, elastic fluids, the constitution of bodies and their chemical synthesis, etc.
On the elemental (atomic) composition of compounds, he says that;
“In order to convey a knowledge of chemical facts and experience, the more clearly, it has been generally deemed best, to begin with, the description of such principles or bodies as are the most simple, then to proceed to those that are compounded of two simple elements, and afterward to those compounded of three or more simple elements….”
History of Dalton Atomic Theory
Alchemy, an ancient science, existed till the late 16th century. It was a series of scientific advancements in the field of science. At that time, chemistry was not even considered a science. It was because alchemy was a spiritual and magical essence, whereas chemistry related to structures and compositions of materials.
The works of Robert Boyle, Antoine Lavoisier, Issac Newton, John Dalton, and Joseph Priestly paved the path for chemistry to be considered a proper science. This time frame exists in both the 17th and 18th centuries when John Dalton carried on the work started by Robert Boyle and Antoine Lavoisier and took it to the development of his atomic theory.
Dalton’s atomic theory is based on the law of conservation of mass and the law of constant compositions. The law of conservation of mass, which was devised by Antoine Lavoisier in 1789, stated that matter is neither created nor destroyed in a closed system. However, the law of constant compositions stated that pure substances or compounds have always the same composition of their elements.
Postulates of John Dalton’s Atomic Theory
There are five basic postulates of Dalton’s atomic theory.
- Each element is made up of tiny and indestructible particles called atoms.
- Atoms of a single element are always identical.
- Atoms of different elements are different in terms of their atomic weights and chemical properties.
- Chemical compounds are formed by the combination of atoms of different elements in specific ratios.
- Atoms can neither be created nor destroyed. They can always be recovered, and never change in a chemical reaction.
Explanation of Postulates
Dalton worked and expanded the work of Leucippus of Miletus (5th-century B.C.E) who assumed that all substances are made up of solid, hard, impenetrable, and mobile particles, which he called “atomos” (literally indivisible) in around 430 B.C.E.
Dalton proposed that every single atom of an element is the same as other atoms of that element. For example, all the atoms present in zinc are the same. He also explained that the atoms of one element are different from the atoms of other elements. For example, the atoms of zinc are different from the atoms of gold, etc. So, different substances necessarily have different boiling points, melting points, and other properties.
The formation of compounds takes place when different types of atoms combine with each other. For example, sodium chloride (NaCl) is formed when sodium (Na) and chloride (Cl) atoms react with each other.
He also suggested that when a chemical reaction takes place between atoms of different elements, the changes occur only in atoms’ arrangements. Atoms are not destroyed or formed during a chemical reaction. This is because they are indivisible.
Limitation of Dalton’s Atomic Theory
Although Dalton’s atomic theory can be regarded as a major step in the development of the first complete atomic theory in terms of atoms and their properties, it also had some shortcomings. These limitations got picked up by the working of modern scientists like; J.J Thompson, Ernest Rutherford, Neil Bohr, etc.
- Dalton’s atomic theory could not explain why elements have different masses, valencies, etc. It means that he had no idea about fundamental particles such as electrons, protons, etc.
- It does not account for isotopes, isobars, isotones, or even allotropes of atoms.
- This theory could not explain why atoms react at all.
- He proposed that elements combine in a simple whole ratio but today, some complex organic compounds show atomic ratios that are not simple whole numbers.
- The existence of matter in three different states (solids, liquids, and gases) and their binding forces could not be explained here by Dalton.
- He also could not make a distinction between particles that take part in chemical reactions( atoms), and the ones that exist independently (molecules).
- Dalton assumed that one atom of hydrogen and oxygen each when combined, constitutes water. Through this and his further studies, he hypothesized that an oxygen atom is 5.6 times heavier than a hydrogen atom.
- He was the one to realize that the rearrangement of individual atoms actually creates new products (compounds).
- There were many elements, that, for the first time, were revealed as compounds by Dalton.
What are the merits of Dalton’s Atomic Theory?
Merits of Dalton’s atomic theory:
- It approves the law of conservation of mass and the law of proportions.
- It explains the difference between atoms, elements, and compounds.
- It provided the complete basic framework for the development of new and advanced theories of atoms.
How does Dalton’s atomic theory differentiate between elements and compounds?
Dalton, in this atomic theory, suggests that;
“Elements are made up of the same atoms while compounds are formed when different types of atoms combine together”.
What are the 5 key postulates of Dalton’s atomic theory?
- Elements are made up of atoms.
- Atoms of an element are identical.
- Chemical compounds are formed by the combination of atoms of different elements.
- Chemical reactions are the result of the rearrangement of atoms.
- Atoms are never created or destroyed. They just change their forms.
What are the key shortcomings of Dalton’s atomic theory?
- Dalton’s atomic theory could not explain the difference between atoms and molecules.
- It does not explain the existence of sub-particles like electrons, protons, and neutrons.
- It has no explanation for isotopes, isobars, isotones, or even allotropes.
- This theory could not account for the reason for atoms to react.
- It also does not account for the reason compounds are in solid, liquid, or gas phases.
- This theory emphasized simple ratios of reacting atoms, which is no longer true.
According to Dalton’s atomic theory, what makes one element different from another?
Accordion to Dalton, elements are made up of small particles. These elements are different from one another in their properties, size, arrangement of atoms, types of atoms, masses, etc.
- A New System of Chemical Philosophy By John Dalton
The Scientist’s Atom and the Philosopher’s Stone: How Science Succeeded and Philosophy failed to gain knowledge of atoms By Alan Chalmers (University of Sydney)