Why did Mendeleev leave some vacant places in his periodic table ?

(a) Why do we classify elements? (b) What were the two criteria used by Mendeleev to classifiy the elements in his periodic table? (c) Why did Mendeleev leave some gaps in his periodic table? (d) In Mendeleev's periodic table, why was there no mention of noble gases like helium neon and argon? (e) Would you place the two isotopes of chlorine, Cl-35 and Cl-27 in different slots because of their different atomic masses or in the same slot because theri chemical properties are the same? Justify your answer.

(a) Why do we classify elements ? (b) What were the two criteria used by Mendeleev in creating his Periodic Table ? (c ) Why did Mendeleev leave some gaps in his Periodic Table? (d) In Mendeleev's Periodic Table, why was there no mention of Noble gases like Helium, Neon and Argon? (e ) Would you place the two isotopes of chlorine , Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same ? Justify your answer.

Why did Mendeleev leave some gaps in his Periodic Table?

Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that?