Question

What are Anomalous pairs of elements?

Answer 1

For answering this question we should learn about Mendeleev’s Periodic Table. There were few Periodic tables before this Modern periodic table was made. First was the Dobereiner triads who arranged the similar type in a group of three and called it triads. Then came Newlands Periodic table; he arranged the elements in rows where one row contained eight elements like notes of music. He said every eighth element will have similar properties to that of the first ; this is known as Newlands law of octaves. This was the first attempt to arrange elements according to atomic numbers.

Complete answer:
Now we talk about Mendeleev’s Periodic table. Mendeleev said that periodic tables are fundamental properties of atomic mass, he compared atomic masses of different elements and arranged them according to their atomic mass in increasing order. He arranged elements in such a way that every eighth element comes below the first element. He called horizontal rows Periods and Vertical rows groups.
There were some elements which are not in order of increasing atomic mass. The element before had more atomic mass than the element after these two elements form a pair of Anomalous pair of elements. there were three pair of anomalous elements in Mendeleev’s table
Argon and potassium: Argon had atomic mass of $40$ whereas Potassium had atomic mass of $39.1$.
Cobalt and Nickel: Cobalt had atomic mass of $58.93$ whereas Nickel had atomic mass of $58.71$.
Tellurium and Iodine: Tellurium had atomic mass of $128$ whereas Iodine had atomic mass of $127$.

Note:
Mendeleev’s Periodic table correctly predicted the position of Inert gases and he put in some vacant spaces for some undiscovered elements, after discovery of new elements it was found that Mendeleev correctly predicted the position of undiscovered elements based on similar behaviour and properties.