ions Of Mendeleev's Periodic Table ✦ Anomalous position of Hydrogen Hydrogen as 1 electron in their outermost shell and alkali metals also have 1 electron in the outermost shell .Therefore many PROPERTIES of hydrogen are same to those alkali metals.Hence , hydrogen can be placed along the group of alkali metal in the Mendeleev's Periodic Table.✦ Position of isotopes { isotopes : those element whose atomic number is same but atomic MASS is different . }The Mendeleev's Periodic table was based on the increasing order of atomic masses and isotopes of the elements have different atomic masses , therefore they should be placed at different position , but due to the same chemical properties they are placed in the same position.Thus , isotopes of the element posed a challenge to Mendeleev's Periodic Table.✦ Different groups of elements within sub groups In the Mendeleev's Periodic Table , elements placed in a group having similar chemical properties . But there is no similarity in the properties of elements in two sub groups.For example , Li , NA , K , Rb , Cs present in group I A are quit different from Cu , Ag , Au in group II B .✦Uncertainity in prediction of new elements In the Mendeleev's Periodic Table , elements are arranged in increasing atomic masses . But the atomic masses do not INCREASE in regular manner in going from one element to the next .Further , the difference in atomic masses between two successive elements is small among lighter elements and large in heavier elements . Therefore , it is not possible to predict how many new elements could be discovered between two known elements especially among heavier elements._______________________________________ ❁ Extra Shots❁☞ Mendeleev is also called as father of Modern Periodic Table .☞ He was the first person to arranged all the 63 elements that were discovered that time into groups { vertical columns } and periods { horizontal rows } .☞ The noble GASES like Ar { Argon } , Ne { Neon } were not known when Mendeleev give his periodic table . These gases were discovered much later by one of the student of Mendeleev , Henry Moseley.________❁❁__________