At the turn of the nineteenth century, about 30 elements were known. 50 years later by the 1850s, scientists had discovered sixty three chemical elements and the numbers kept increasing.
In the famous atomic theory of John Dalton (1805), it was suggested that the atoms of an element have a characteristic mass. So, attempts were made to classify elements on the basis of their atomic masses.
Mendeleev classified the then known 56 elements on the basis of their physical and chemical properties in the increasing order of the atomic masses, in the form of a table.
The modern periodic table is also known as the long form of the periodic table or the extended form of the periodic table.
Members of the same group have similar electronic configuration of the valence shell and thus show same valency.
The number of elements in these periods is based on the way electrons are filled into various shells.
The distance from the centre of the nucleus to the outermost shell of an atom is called the atomic radius of that atom.
While studying the periodic table, it will be noticed that with the increase in the atomic number, there is an increase in atomic mass as well (with some exceptions).
The earliest classification was into metals and non-metals, which was on the basis of physical and chemical properties.