(a) State any three limitations of Mendeleev’s classification of elements. (b) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table ? (c) Which group of elements was missing from Mendeleev’s original periodic table?

(a) Limitations of Mendeleev’s classification of elements:

(i) It failed to explain the position of isotopes.

(ii) The wrong order of atomic masses of some elements could not be explained.

(iii) The position for hydrogen could not be assigned in the periodic table.

(b) Silicon and Germanium.

(c) Noble gases were missing from Mendeleev’s original periodic table.