Question

Which of the following is not an anomalous pair?
(This question has multiple correct options)
A.$S,Cl$
B.$Te,I$
C.$Co,Ni$
D.$Ar,Ne$

Answer 1

Deviating from the general one or deviating from the common order is called an anomalous. As we know that the Dmitri Mendeleev formulated the periodic law and he arranged the elements with similar properties and Mendeleev observed that most of the elements placed in the periodic table were arranged in the order of increasing atomic masses.

Complete answer:
We need to remember that Dimitri Mendeleev arranged the elements with their increasing atomic masses. But if you see the periodic table some of the elements placed are inconsistent, like, the increasing mass number not obeyed. They are called anomalous pairs.
The mass of an element is called atomic mass, the unit is $amu$.
Let us see the options one by one to find which not an anomalous pair are.
Sulphur $(S)$ atomic mass is $32.064amu$ and Chlorine $(Cl)$ atomic mass is $35.453amu$ . Sulphur was placed right before chlorine in the periodic table with increasing atomic mass.
Tellurium $(Te)$ atomic mass is $127.60amu$ and Iodine $(I)$ atomic mass is $126.90amu$ . Tellurium was placed before iodine in the periodic table but the atomic mass of tellurium is higher than iodine.
Cobalt $(Co)$ atomic mass is $58.933amu$ and nickel $(Ni)$ atomic mass is $58.699amu$ . Cobalt has a higher atomic mass than nickel but it is placed after cobalt.
Argon $(Ar)$ atomic mass is $39.948amu$ and Neon $(Ne)$ atomic mass is $20.179amu$ . Neon was correctly placed in the periodic table.
From the above information, $Te$ , $I$ and $Co$ , $Ni$ are anomalous pairs.
So, the correct answer is “Option A and D”.

Note:
We have to remember that at the time Mendeleev arranged the elements in the periodic table contains $63$ but the modern periodic table contains $118$ elements. Mendeleev left vacant places for undiscovered elements. Periodic table contains seven periods and eighteen groups, if new elements are discovered in the future, that will be placed in the eighth period.