Why do we classify elements? (b) What were the two criteria used by Mendeleev to classify 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-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.

• The elements are classified according to the properties possessed by the elements. Similar properties falls in same group and thus large number of elements reduced to a few group and orderly arranged. (b)(i) Increasing atomic masses

(ii) Grouping together of elements having similar properties.

• In order to make sure that the elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table.
• The noble gases were not known at that time so there was no mention of noble gases like helium, neon and argon in Mendeleev’s periodic table.
• Because their chemical properties are The two isotopes of chlorine have the same atomic number and chemical properties. The arrangement of elements is on the basis of increasing atomic

number and the chemical properties depend on the atomic number.