Question

(a) State Mendeleev's periodic law.
(b) What chemical properties of elements were used by Mendeleev in creating his periodic table?
(c) State any three limitations of Mendeleev's classification of elements.
(d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table?
(e) Which group of elements was missing from Mendeleev's original periodic table?

Answer 1

(a) Mendeleev's periodic law: According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means, when the elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at a regular interval of the periods.

(b) Mendeleev used the formula of the oxides and hydrides produced by the elements as the basic chemical properties of elements, in creating his periodic table.

(c) Limitations of Mendeleev's classification of elements are:

i) Mendeleev's periodic law could not explain the position of isotopes in the periodic table.
ii) Mendeleev's periodic law could not assign a proper position to the element hydrogen in the periodic table.
iii) Mendeleev's periodic law failed to explain the wrong order of atomic masses of some elements.

(d) Germanium (Ge) and scandium (Sc) are the two other elements besides gallium, which have since been discovered, for which Mendeleev had left gaps in his periodic table.

(e) Group 18 consisting of noble gases was missing from Mendeleev's original periodic table as the noble gases were not known at that time.