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, Cl-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) The elements are classified into groups so that the elements with similar properties fall in the same group and hence the study of a large number of elements is reduced to the study of a few group of elements.

(b) The two criteria used by Mendeleev to classify the elements in his periodic table are:

(i) Increasing atomic masses.

(ii) Grouping together of elements having similar properties.

(c) 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.

(d) Noble gases are inert and are present in extremely low concentrations in the atmosphere. Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev's table.

(e) The isotopes of chlorine, Cl-35 and Cl-37 are placed in the same slot because they similar chemical properties and same atomic number.