John Dalton (1766-1844)

Our modern understanding of matter can be traced back to the atomic theory of John Dalton (1766-1844), an English Chemist. Dalton was interested in the properties of gases, particularly the experiments of Robert Boyle (1627-1691) of which Boyle's Law is probably familiar to you. Dalton thought that all gases consisted of tiny particles and he contributed to the theory of gases by founding a law which we know today as Dalton's law of partial pressures. Dalton went further and asserted that all matter and not just gases must consist of small particles. Relating his atomic theory to Democritus, he used the name "atom" to name these tiny particles of matter.

Dalton's atomic theory held that all substances are composed of atoms in different proportions. All the atoms of one element, said Dalton, were exactly identical and the atoms of each element were different to the atoms of every other element. Furthermore, the atoms differed from each other only in mass. Now this last feature was something that could be measured experimentally from the masses of the chemical elements known at that time.

However, it was Dimitri Mendeleev (1834-1907) a Russian chemist who brought order to Dalton's atomic theory. When Mendeleev placed the elements in order of atomic weights he noticed that the properties of valence, or the ability that each type of atom has for combining with other atoms, had a periodic nature. For example the first atom in Mendeleev's list was lithium which had a valence of one. It combines with only one other atom. Beryllium has a valence of two - it combines with as many as two different atoms. Next is Boron with a valence of three and then carbon with a valence of four. Mendeleev found that he could arrange all the sixty-three elements known in the table in order of their atomic weight and get periodic rise and fall of valence i.e. 1,2,3,4,3,2,1, etc. He then arranged them in rows under each other so that elements with the same valence fell in a vertical column. Mendeleev saw that when the elements were placed in this way, they tended to show similar chemical properties and it is because of this periodicity that Mendeleev's table is called the periodic table. Mendeleev published his table in 1869.