Question

(a) 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, CI-35 and CI-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.

Answer 1

(a) Elements are classified into certain groups in such a manner that elements belonging to the same group exhibit similar properties. This eases the study of elements, as we can reduce the study to a few groups of elements rather than studying the properties of all the 115 elements known at present, which is very difficult. This is the reason for classification of elements.

(b) The two criteria used by Mendeleev to classify the elements in his periodic table are:
(i) Increasing atomic masses
(ii) Grouping of elements which exhibited similar properties, placed under the same vertical column (group).

(c) Mendeleev left some gaps in his periodic table for the elements which were not known at that time, to ascertain that elements having the same properties fell in the same vertical column or group, without disturbing the arrangement of previous elements.

(d) There was no mention of noble gases like helium, neon and argon in Mendeleev's periodic table as those gases were not known at that time.

(e) The isotopes of chlorine Cl-35 and Cl-37 should be placed in the same slot because their chemical properties are the same. The two isotopes of chlorine have the same atomic number and chemical properties. We know that the arrangement of elements is made on the basis of atomic number in the periodic table. And chemical properties of the element depend on the atomic number of the atom.