Mendeleev’s periodic table was the first systematic approach to the classification of elements reflecting their properties. But it was applicable only to sixty-three elements known at that time and also had several defects. Since, then new elements have been, either discovered or synthetically prepared. Total number, of elements, known now is 118. Mendeleev’s table would not hold all the elements.
Atoms of elements are, shown experimentally to consist of fundamental particles of proton, electron, and neutron. Rutherford proved that every elemental atom is, made up of a fundamental positively charged particle -namely ‘proton’.
Henry Moseley, from his atomic spectral analysis, observed that spectral lines are, related to the number of positively charged particles (Proton) present in the nucleus of the atom. Atoms are, considered unique by the total number of protons present in the nucleus. So, the number of protons or ‘atomic number'(as used to be called) was considered as the fundamental nature and not the atomic weight (as proposed by Mendeleev) of elemental atoms. A new classification was, developed based on this atomic number and came to be, known as modern periodic table.