The atomic model of Dalton represents the atom as the smallest and indivisible particle of matter . John Dalton (1766-1844) proposed that atoms were the basic building blocks of matter and represented them as solid spheres.
The idea that matter was composed of very small particles that could no longer be divided was initially considered in the fifth century BC. of C. by Democritus. However, it took more than 20 centuries for the notion of the atom to be accepted.
Dalton made the first scientific presentation of the atom in 1808 . Subsequently, this atomic model was displaced as knowledge and technology advanced.
Postulates of Dalton’s atomic theory
The postulates of Dalton’s atomic theory are inferred from his research on the atom. Below, we explain each of your proposals.
Each element is made up of tiny particles called atoms
The best way to explain the behavior of gases according to Dalton was to assume that the elements were composed of atoms.
The atoms of an element are equal
Unlike many of his contemporaries, Dalton thought that the atoms of an element were equal and that each element must have its own atoms. For example: iron (Fe) had iron atoms, which were different from the atoms of the element silver (Ag).
Chemical compounds are formed when the atoms combine
An atom of a substance X combines with an atom of substance Y to form compound XY. In the case of carbon monoxide CO, a carbon atom C is combined with an oxygen atom O.
Chemical reactions are produced by the rearrangement of atoms
When the compounds react, a rearrangement of the atoms occurs. For example, if a compound XY reacts with a Z element, two new compounds may occur: XZ or YZ.
The atoms do not change
For Dalton, the atoms were indestructible and could not change with each other.
How did Dalton come to the atomic theory?
Dalton was a professor at a University of Manchester (England), interested in meteorology. Studying the nature of air, Dalton proposed in 1803 the law of partial pressures of gases. He thought that the gases consisted of small particles that attracted and rejected each other.
In 1804 he proposed the law of multiple proportions, according to which a compound is constituted by a fixed and proportional number of elements.
Although Dalton published his work partially in scientific journals of the time, all the information was compiled in 1808 in the book A new system of chemical philosophy, where he explained in detail his findings.
Failures of Dalton’s atomic theory
Evidently for our current knowledge, Dalton’s theory presents many flaws. Below we explain, as a summary, each of the fundamental aspects of Dalton’s theory that are rejected.
The atom is not indivisible
The atom, in fact, is made up of many other subatomic particles. It took almost a hundred years after Dalton’s theory to discover the electrons and protons, which made it clear that the atom was indivisible.
The atoms do change
An atom can change due to radioactivity. When unstable atoms lose particles, they can give rise to a completely new element. For example: uranium -238 is transformed by radioactive decay into thorium-234.
Water is not the combination of a hydrogen and an oxygen
Now we know that the water molecule consists of two hydrogen atoms and one oxygen atom. Dalton had an error in the calculation of water.
The atomic model that followed that of Dalton was that of “pudin with raisins”, proposed by JJ Thomson (1856-1940), where the electrons (raisins) were embedded in a positive mass (pudding).
Resistance to Dalton’s atomic theory
The British chemist Sir Henry Enfield Roscoe (1833-1915) mocked atomic theory by saying that atoms were bits of wood invented by Mr. Dalton.
It probably referred to the wooden construction models that some scientists used to represent different types of atoms.