The periodic table displays all chemical Elements systematically in order of increasing atomic number,i.e., the number of protons in the nucleus.In 1869, Russian chemist Dmitri Mendeleev first described an arrangement of the chemical elements now known as the periodic table. Mendeleev wrote, "The elements, if arranged according to their atomic weights, show a distinct periodicity of their properties.... Elements exhibiting similarities in their chemical behavior have atomic weights which are approximately equal (as in the case of Pt, Ir, Os) or they possess atomic weights which increase in a uniform manner (as in the case of K, Rb, Cs)."
isotopes of an element have the same number of protons but a different number of neutrons.They are chemically very similar but have a different atomic masses.