Question

Anomalous pair among the following is:
A. Boron- silicon
B. aluminium- nickel
C. beryllium- indium
D. cobalt-nickel

Answer 1

Mendeleev arranged the elements in order of increasing relative atomic mass. When he did this, he noted that the chemical properties of the elements and their compounds showed a periodic trend. There are six periods (numbered from 1 to 6) or horizontal rows in the Mendeleev’s periodic table. Periods 4, 5, and 6 are divided into two halves.

Complete step by step answer:
-Mendeleev’s periodic table simplified the study of elements. As the arrangements of elements showing properties were classified, it was very useful in studying and remembering the properties of a large number of elements in a systematic way.
-The discovery of isotopes was one of the limitations of Mendeleev’s periodic table.
-Since Mendeleev’s periodic table was based on atomic masses of elements, so the isotopes should be placed in different columns even though they represent the same element and hence, isotopes haven’t been given a separate place in the periodic table.
-In the Mendeleev periodic table, the anomalous pair means the pair of the element where the mass of the first element is higher than that of the second element in the pair. In this case cobalt-nickel, the mass of cobalt is $58.9332\,gm/mol$ greater than the nickel with the mass $58.70gm/mol$. That is why this pair is called an anomalous pair.

So, the correct answer is D.

Note:
Mendeleev’s periodic table became widely accepted, primarily because he predicted the characteristics and placement of elements that were yet to be discovered. He left gaps in his table to place the elements not known at that time. By looking at the physical and chemical properties of the elements, next to a gap, he could also predict the properties of these undiscovered elements.