Nowadays we all know that matter is made up of small particles called molecules which in turn are made up of atoms of different elements (which are currently known to be made up of different subatomic particles such as protons, neutrons and electrons).

But although precursors already existed even in ancient Greece, it would not be until 1803 that the theory that matter is formed by basic and indivisible units that unite to form different compounds, would be elaborated at a scientific level, considering these indivisible units and compound atoms, or what Avogadro would later call molecules, the compounds formed through them.

That is the Dalton’s Atomic Theory , which elaborated different postulates or principles that tried to give an explanation to the configuration of matter.

John Dalton: A Brief Introduction to the Creator of Atomic Theory

The figure of John Dalton is widely known as he was the founder of atomic theory and also for investigating and making known the alteration of vision known as colour blindness, which he also suffered from. This scientist was born in Great Britain in 1766, the son of a hard-working family with few resources. In spite of the difficulties, Dalton would learn science and mathematics at school and even teach at the age of twelve. Eventually he would open and run a school, along with his brothers.

Later on he expanded his interest to various sciences such as astronomy and geography , even giving lectures on the subject. He considered studying medicine, but would be discouraged by his environment. He carried out various investigations and publications in different areas, among them meteorology or even grammar. One of the best known is that which has to do with the lack of colour perception that he himself suffered and which is now known as colour blindness.

I would also investigate other phenomena such as heat, the behaviour of gases and different elements. His work in the latter areas would lead him to reflect on the composition of matter, which would eventually lead to the development of atomic theory.

Dalton’s Atomic Theory

One of Dalton’s most important and recognized contributions to the field of science is his conception of atomic theory. This theory established a model that tried to explain the behavior of matter as well as the fact that the combination of different proportions of different substances can generate different compounds, explaining the composition of complex elements from different balances of other substances.

The various investigations that Dalton would carry out would lead him to think that all matter is made up of compounds and elements , the former being formed by combinations of the latter. There are a series of indivisible particles, the so-called atoms, which form the basis of the different particles and have different characteristics from one another. Each of the elements is formed from atoms of different kinds. Dalton’s theory recovers concepts that are typical of ancient philosophers, such as the concept of the atom in the Greek Democritus, but with a somewhat different meaning from the original.

Thus, Dalton did not believe that all matter could be identified with a single substance, but that there were atoms of different types and characteristics, weight being one of the most studied variables . In fact, the creator of the atomic theory established a table of elements based on the weight attributed to each of the known types of atoms, such as hydrogen and oxygen (although this initial table was not correct due to the lack of knowledge and difficulty of measuring the weight of the different particles with the techniques of that time). It is also due to him that hydrogen is considered the basic standard when considering the atomic mass of each element, as it is the lightest element.

Dalton’s postulates

The Dalton atomic theory can be summarised on the basis of a series of postulates , which are described below.

  1. Matter is made up entirely of atoms , units of indivisible matter that can neither be generated nor destroyed.
  2. The atoms of a particular element have in all cases the same size, weight and qualities , being all equal to each other.
  3. The characteristics of the atoms of different elements are also always different , possessing different characteristics.
  4. Although they can combine with other substances to form different compounds, the atoms themselves remain unchanged , and cannot be split or destroyed.
  5. Compounds are formed by combining the atoms of two or more different elements.
  6. The combination of different types of atoms is carried out through simple relationships .
  7. The combination between two specific elements can give rise to different compounds depending on the proportions in which they are mixed .
  8. In chemical reactions it is possible to create, dissociate or transform molecules , being these a reorganization of the atoms that compose each compound.
  9. The same compound is always produced by the same proportion in the combination of the atoms.

Some aspects that the most current evidence has contradicted

Dalton’s atomic theory has been one of the most important in science in describing the structure of matter. However, since the time when this theory was developed there have been numerous advances that have shown that some of the postulates defended by the author are not true.

For example, the fact that the atom is a basic and indivisible unit has been shown to be false, as it can distinguish within the atom different parts formed by subatomic structures such as protons, neutrons and electrons .


The fact that all atoms of the same substance have the same properties has also proved to be uncertain. We can find atoms of different electrical charge according to the balance between protons and electrons (what we know as ions), as well as different atomic masses of the same element (isotopes).

A third aspect that has proven to be divergent with Dalton’s atomic theory is the fact that atoms are unchangeable, something that has been denied with the advent of fusion and nuclear fission .

Although the evidence has shown that some of the postulates are not entirely true, Dalton’s theory has laid the foundations of modern chemistry and allowed a breakthrough in the understanding of matter and its behaviour.

Bibliographic references:

  • Gribbin, J. (2006). History of science: 1543-2001 (2nd edition). Barcelona: Crítica, S.L.
  • Petrucci, R.; Harwood, W.; Herring, G. & Madura, J. (2007). General Chemistry. 9th ed. Upper Saddle River, New Jersey: Pearson Prentice Hall.
  • Rosental, M. and Iudin, P. (1973). Philosophical Dictionary. Ediciones Universo.
  • Soledad, E. (2010). Chemistry and atomic theory. General chemistry. UNED, 22-23.