In the 5th century B.C. the Greek philosopher Democritus expressed the idea that all matter was made up of many small, indivisible particles which he called atoms (meaning indestructible or indivisible). Although Democritus’ idea was not accepted by many of his contemporaries (among them Plato and Aristotle), it was maintained. Experimental evidence from some scientific investigations supported the concept of “atomism”, which gradually led to the modern definitions of elements and compounds. In 1808, an English scientist, Professor John Dalton, formulated a precise definition of the indivisible units of which matter is composed and which we call atoms.
Dalton’s work marked the beginning of the era of modern chemistry. The hypotheses about the nature of matter, on which Dalton’s atomic theory is based, can be summarized as follows:
The elements are made up of extremely small particles called atoms. All atoms of the same element are identical, have the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements.
Compounds are made up of atoms of more than one element. In any compound, the ratio of the number of atoms between two of the elements present is always a whole number or a single fraction.
A chemical reaction involves only the separation, combination or rearrangement of atoms; it never involves the creation or destruction of atoms.
Dalton’s concept of an atom is much more detailed and specific than Democritus’ concept. The first hypothesis states that the atoms of one element are different from the atoms of all other elements. Dalton did not attempt to describe the structure or composition of atoms. Nor did he have any idea what an atom looked like, but he realized that the difference in properties exhibited by elements such as hydrogen and oxygen could only be explained from the idea that hydrogen atoms are different from oxygen atoms.
The second hypothesis suggests that, to form a given compound, not only are atoms of the right elements needed, but a specific number of those atoms is indispensable. This idea is an extension of a law published in 1799 by the French chemist Joseph Proust. Proust’s law of definite proportions states that different samples of the same compound always contain the same elements in the same mass ratio. Thus, if samples of carbon dioxide gas obtained from different sources are analyzed, the same mass ratio of carbon and oxygen will be found in all samples. Then, if the ratio of the masses of the different elements in a compound is a fixed quantity, the ratio of the atoms of the elements in that compound must also be constant.
Dalton’s second hypothesis confirms another important law, the law of multiple proportions. According to this law, if two elements can combine to form more than one compound, the mass of one of the elements combining with a fixed mass of the other maintains a ratio of small integers. Dalton’s theory explains the Law of Multiple Proportions in a very simple way: different compounds formed by the same elements differ in the number of atoms of each kind. For example, carbon forms two stable compounds with oxygen, called carbon monoxide and carbon dioxide. Modern measurement techniques indicate that one atom of carbon combines with one atom of oxygen in carbon monoxide, and with two atoms of oxygen in carbon dioxide. Thus, the ratio of oxygen in carbon monoxide to carbon dioxide is 1 : 2. This result is in agreement with the law of multiple ratios.
Dalton’s third hypothesis is a way of stating the law of conservation of mass, which states that matter is neither created nor destroyed. Since matter is made up of atoms, which do not change in a chemical reaction, it follows that mass must also be conserved. Dalton’s brilliant idea about the nature of matter was the main stimulus for the rapid progress of chemistry during the 19th century.
