The elements are the basic units of all types of matter. In 1800, only 31 elements were discovered. Fifty years later by 1850, scientists had discovered 63 chemical elements and the numbers kept increasing. With the discovery and study of more and more elements and their compounds, the various data about them also increased. It became progressively difficult to organise all the known elements but different scientists made attempts to look for some trends and patterns in their properties. Few important attempts for the classification of elements are described above.
Dmitri Ivanovich Mendeleev, a Russian chemist, in 1869, delivered a remarkable contribution to arrange chemical elements in the Periodic table. He studied the chemical properties of a large number of the elements and stated that the chemical properties of the elements are the periodic function of their atomic masses. The work was published in Zeitschrift für Chemie. The law became popular as Mendeleev's Periodic Law.
In 1913, Henry Moseley, an English physicist, by his experiment concluded that atomic number is a more fundamental property of an element and not its atomic mass. Thus, he defined Modern Periodic Law as- The physical and chemical properties of the elements are the periodic functions of their atomic numbers.
The principles of classification of elements are: Classification based on differentiating electrons and Classification based on a number of incomplete shells. These principles are discussed above. The periodic table is divided into four main blocks (s, p, d, and f) depending upon the sub-shell to which the differentiating configurations of the atom (the valence electron, which differentiates it from the previous atom is called the differentiating electron).
The properties which are directly or indirectly related to their electronic configuration and show gradual change when we move from left to right in a period or from top to bottom in a group are called periodic properties. Such properties show a variation with change in atomic number. Some important chemical properties that exhibit periodicity are electronic configuration, ionization energy, electron affinity, electronegativity, metallic character, nature of oxides, oxidation state, and reducing character etc. (Specific heat, refractive index etc. are not periodic properties.)