$(a)$ 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.

(N/A) Classification of elements helps in:
$(i)$ Systematic study of the properties of elements.
$(ii)$ Making it easier to understand and remember the properties of elements.
$(b)$ The two criteria used by Mendeleev were:
$(i)$ Increasing order of atomic mass.
$(ii)$ Similarity in the chemical properties of the elements.
$(c)$ Mendeleev left gaps in his periodic table for elements that were yet to be discovered,such as gallium and germanium.
$(d)$ There was no mention of noble gases like helium,neon,and argon because 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 in different positions because the basis of classification was the increasing order of atomic masses. However,since their chemical properties are identical,they are placed in the same position in the modern periodic table,which is based on atomic number.