Who devised the modern periodic table?
At the time the periodic table was first devised, there were many elements that had not yet been discovered. Their existence could be inferred, but they couldn't actually be put into the table until they were discovered. When the periodic table was first devised, there were lots of elements that had not been discovered yet.
Numerous forms of Periodic Table have been devised from time to time. Some forms emphasise chemical reactions and valence, whereas others stress the electronic configuration of elements. A modern version the so called long form of the periodic table of the elements, is the most convenient and widely used.
The mendeleevs periodic table was classified according to the atomic weight of elements and the modern periodic table is classified according to atomic no. Mendeleeves peiodic table failed as there was no space for isotopes in his periodic table which have different atomic weight. This problem wont occur in modern peridic table bcoz isotopes have same atomic no.