Question

Anomalous pair in Mendeleev’s periodic table is:
A. Li, Na
B. Mg, Al
C. Co, Ni
D. Be, B

Answer 1

Hint: Mendeleev’s considered atomic mass as fundamental property of an element. In Mendeleev’s Periodic table, the horizontal rows are called periods and vertical columns are called groups. Anomalous pairs are those pairs of elements in which the increasing order of atomic masses was not obeyed.

Step by step answer: Let us discuss about Mendeleev’s periodic table in detail:
Mendeleev’s classified periodic tables as fundamental properties of atomic masses and he compared atomic masses of different elements to arrange them in periodic tables. He also studied their chemical and physical properties. While studying the chemical properties of the elements he majorly studies the compound formed by the reaction of elements with hydrogen and oxygen because mostly every element reacts with these elements.
He arranged the elements in increasing order of the atomic masses and in such a way that every eighth element comes below the first element.
Horizontal rows are called periods and vertical columns are called groups. There are seven periods and eight groups.
He got success in placing all the elements in the periodic table. The vacant places present in the periodic table correctly predicted the existence of undiscovered elements. Based on their position it becomes possible to predict the properties of undiscovered elements before their actual discovery. E.g. Predicted Elements were Eka-boron, Eka-aluminium , Eka-silicon. After discovery these elements are named Scandium, Gallium and Germanium respectively. The properties of the predicted element and the discovered element were almost the same. E.g. Predicted atomic weight of Eka-aluminium was 68 and the discovered element Gallium had atomic weight 69.72.
There is a demerit is that at some places elements with high atomic mass are placed before low atomic mass elements.
For example, Cobalt with atomic mass 58.93 was placed before Nickel with atomic mass 58.71 and so it became the anomalous pair in the Mendeleev’s periodic table.
Therefore the anomalous pair in Mendeleev’s periodic table is Cobalt and Nickel.

Hence our correct option is : C

Note: Remember there are some demerits also of Mendeleev’s periodic table like it failed to classify hydrogen and also failed to give position of isotopes.The meaning of anomalous itself tells about deviation.