1) Mendeleeff arranged all then known elements in a systematic order in the increasing order of their atomic weights.
2) He tried to explain the similarities of elements in the same group in terms of their common valency.
3) Same group of elements have similar outer electronic configuration having same valence electrons.
4) Elements show similar chemical properties in a group.
5) That is why he can arrange the elements nearly close to the arrangements in the modem periodic table.