Question

In the periodic table as formulated by Mendeleev, which of the following constitutes an anomalous pair of elements?
(a)- H and He
(b)- Br and Ne
(c)- K and Ar
(d)- Na and B

Answer 1

Hint: Mendeleev proposed the periodic law: the physical and chemical properties of elements are a function of their atomic weights. But this periodic law has some defects like anomalous pairs of elements. Some elements with higher atomic weights precede the element with lower atomic weight.

Complete answer:
The first real breakthrough in the classification of elements was provided by a Russian chemist, Dmitri Ivanovich Mendeleev.
He proposed the periodic law, The physical and chemical properties of elements are a particular function of their atomic weights, i.e., when the elements are arranged in order of their increased atomic weights, elements with similar properties are replaced after certain regular intervals. This repetition of properties of elements after certain regular intervals is called periodicity of properties.
Despite the advantages of Mendeleev's periodic table suffers from many serious defects:
The anomalous position of hydrogen: Hydrogen is placed in group IA. However, it resembles the elements of both the group IA (alkali metals) and group VIIA (halogens). Therefore, the position of hydrogen in the periodic table is not fixed.
Anomalous pairs of elements: Some elements with higher atomic weight precede (are placed before) the elements with lower atomic weights. For example, Ar (at. weight = 39.9) precede potassium (at. weight = 39.1); Co (at. weight = 58.9) precedes Ni (at. weight = 58.7); and Te (at. weight = 127.6) precedes I (at. weight = 126.9).
So, the correct answer is “Option C”.

Note: Some other defects of the Mendeleev periodic table are isotopes of an element that have not been given different places in the periodic table, some similar elements are placed in different groups and dissimilar groups were laced together, the position of lanthanoids and actinoids were not given, etc.