||Every element corresponds to an assembly of a number of protons forming the core of the atom and a number of electrons situated in one shell or more shells of different diameters round the protons. The number of protons determines the mass of the atom. Each element has its own atomic weight, generally different from every other element. Early chemistry recognized that certain elements had similar but slightly different characteristics, for example sodium, potassium and caesium or calcium, barium and strontium, but no general pattern for all the elements was found until Stanislav Cannizzaro (1826 - 1910) who arranged the 60 elements known at the time (1858). When the elements were arranged in order of increasing atomic weight, a curious repetition of chemical properties at regular intervals was revealed. This was noted by the English chemist , John Newlands (1838 - 1898), but his theories were not generally accepted. It was left to the Russian chemist , Dmitri Ivanovich Mendeleyev (1834 - 1907) who in 1869 put forward his theories and was rightly credited as the true discoverer of the Periodic Table.
Mendeleyev wrote the names and some of the main features of the elements on cards. While arranging this pack of cards in different ways he stumbled upon the pattern which we now recognize as the Periodic Table.
The Periodic Table is an arrangement of elements according to the number of electrons in their shell. Travelling horizontally along the table, the next element gains an electron, while travelling vertically downwards means that the next element gains an outer shell. Elements in the same groups (groups run vertically in the table) exhibit similar properties.
Mendeleyev\'s talent lay in the fact that he knew that there was an underlying trend to the known elements; he did not design the Periodic Table, he discovered it. If his theories were correct, he knew that there were more elements to be discovered where there were spaces in his Periodic Table.
Since Mendeleyev published his table in 1869 a further 40 elements have been discovered or produced by nuclear reactions, and the table has been redesigned in order to accommodate them. AA
Further reading I. Asimov, A Short History of Chemistry.