Dalton’s atomic model represents the atom as the smallest and most indivisible particle in 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 made up of tiny particles that could no longer be divided was initially considered in the 5th century BC. 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 . Later, this atomic model was displaced as knowledge and technology advanced.
Dalton’s Atomic Theory Postulates
Dalton’s postulates of atomic theory are inferred from his research on the atom. Next, we explain each of their proposals.
Each element is made up of tiny particles called atoms
The best way to explain the behavior of gases according to Dalton was assuming that the elements were made up of atoms.
Atoms of an element are the same
Unlike many of his contemporaries, Dalton thought that the atoms of an element were the same and that each element should have its own atoms. For example: iron (Fe) had its own atoms of iron, which were different from the atoms of the element silver (Ag).
Just like these crystal balls, all the atoms of an element, according to Dalton, are the same.
Chemical compounds are formed when atoms combine
An atom of substance X combines with an atom of substance Y to form compound XY. In the case of CO carbon monoxide, a C carbon atom combines with an O oxygen atom.
Chemical reactions are produced by the rearrangement of atoms
When the compounds react, a rearrangement of the atoms occurs. For example, if an XY compound reacts with a Z element, two new compounds can occur: XZ or YZ.
Atoms do not change
For Dalton, the atoms were indestructible and could not change each other.
How did Dalton get to 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 the partial pressures of gases. He thought 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 made up of a fixed and proportional quantity of elements.
Although Dalton partially published his work in scientific journals of the time, all the information was collected in 1808 in the book A new system of chemical philosophy, where he explained his findings in detail.
Failures of Dalton’s atomic theory
Obviously to our current knowledge, Dalton’s theory has many flaws. Below we explain, by way of summary, each of the fundamental aspects of Dalton’s theory that are rejected.
The atom is not indivisible
The atom is actually made up of many other subatomic particles. It took nearly a hundred years after Dalton’s theory to discover the electrons and protons, thereby rendering the atom indivisible.
Atoms do change
An atom can change due to the effect of radioactivity. When unstable atoms lose particles, they can give rise to a whole 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
We now know that the water molecule is made up of two hydrogen atoms and one oxygen atom. Dalton had an error in calculating the water.
The atomic model that followed Dalton’s was “raisin pudding”, proposed by JJ Thomson (1856-1940), where the electrons (raisins) were embedded in a positive mass (pudding).
See also Atom .
Resistances to Dalton’s atomic theory
British chemist Sir Henry Enfield Roscoe (1833-1915) scoffed atomic theory by saying that atoms were bits of wood invented by Mr. Dalton.
It was probably referring to the wooden construction models that some scientists used to represent different types of atoms