The theory that all matter is made of very small particles called atoms which cannot (by chemical means) be broken down into smaller units is not well established. It is, however, little more than a hundred and ninety years since John Dalton first propped his atomic theory.
Philosophers of former civilizations, in particular the Greek thinker, Demokritos (460 – 370 B.C.) had notions that all matter was made of some kind of elementary particles or atoms. So the concept of atoms was by no means new, but it fell to John Dalton to develop the idea. By defining in more detail what he understood by atoms, John Dalton cleared the way for those chemists who followed him to gain a better idea of the constitution of chemical substances and of the mechanism of chemical reactions.
John Dalton included in his theory some important new ideas. He said that the atoms of any one element are identical in all respects and in particular they have exactly the same mass. Furthermore, different elements have atoms of different mass, the mass of its atoms being a characteristic of each element. John Dalton also stated that when chemical combination occurs, small whole numbers of atoms join together.
Except for the slight modification which became necessary when isotopes were discovered, this theory is still accepted today. The various points are, however, more familiar as the basic chemical laws of Conservation of Mass, Constant composition and Multiple Proportions.
I was during the first decade of the nineteenth century that John Dalton arrived at his atomic theory. At the time it was virtually impossible to make accurate measurements because the apparatus available was still quiet primitive. It is all the more remarkable; therefore, that John Dalton’s theory has withstood the test of time. Although he carried out a whole series of experiments to test the various parts of his theory, the experimental errors involved were so large that he could not claim to have proved it conclusively.
Another valuable contribution which John Dalton made to chemistry was his idea for representing chemical compounds visually. A distinctive circular symbol was used to denote the atoms of each element – hydrogen was a circle with a dot in the middle, while a circle with one vertical line through it denoted a nitrogen atom.
John Dalton was born of a Quarker family at Eaglesfiled, a small village in the English Lake District. In 1776, when only ten years old, he entered the service of Elihu Robinson, a wealthy Quaker, who taught him mathematic. In 1781, after a brief spell of teaching in the village school, he joined his brother who was a master at a school in Kendal.
It was during this period that he commenced a journal of meteorological observations which he kept up for the remainder of his life. He also collected butterflies and amassed a vast number of dried plants.
In 1793 he moved to Manchester. At first he taught mathematics and natural philosophy at New College, but after six years he resigned. Thereafter he devoted his life to research which he financed by giving private tuition.
John Dalton Died in 1844 having made one of the greatest contributions to the advancement of theoretical chemistry. He had transformed a vague hypothesis into a definite theory.