Why do we classify elements?

(a) Why do we classify elements ?
(b) What were the two criteria used by Mendeleev in creating his Periodic Table ?
© 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.

(a) Classification of elements helps in:
(i) Systematic study of the properties of elements.
(it) Easy to understand and remember the properties of elements.
(b) The two criteria used by Mendeleev were: (i) Increasing order of atomic mass.
(if) Similarity in the properties of the elements.
© Mendeleev left gap in his periodic table for the undiscovered elements like germanium, gallium.
(d) There was no mention of noble gases like helium, neon and argon as they were not known at that time and were discovered much later.
(e) According to Mendeleev’s periodic table Cl-35 and Cl-37 should be placed at different positions as the basis of classification is increasing order of atomic masses but since their chemical properties are same they should be placed in the same position in the periodic table.