Question

Which are not anomalous pairs of elements in the Mendeleev’s periodic table?
A.Ar and K
B.Co and Ni
C.Te and I
D.Al and Si

Answer 1

Hint: Anomalous in simple terms can be defined as inconsistent or deviating from what is usual, normal, or expected. Check out this inconsistent behaviour of elements from Mendeleev's table and figure out the answer.

Complete step by step answer:
Mendeleev arranged the elements known at the time in order of relative atomic mass. But when he arranged these elements they got arranged in such a way that some inconsistent trends were there.
When we study the Mendeleev’s periodic table we will observe that there is some anomalous behaviour in it. Such as some elements with higher atomic weight were placed before low atomic weight elements in order to maintain similar chemical nature of elements and are called inverted pairs or anomalous pairs.
Argon and Potassium (Ar and K):- Argon with atomic mass of 40 amu was placed before Potassium with atomic mass of 39.1 amu.
Cobalt and Nickel (Co and Ni):- Cobalt with higher atomic mass of 58.9 amu was placed before Nickel with atomic mass of 58.7 amu.
Tellurium and Iodine (Te and I):- Tellurium has mass 128 amu but it was placed before Iodine of atomic mass 127amu.
So our option A, B and C are anomalous pairs, so these options are incorrect.
Now option D, which is Aluminium and Silicon. Aluminium and silicon are placed right after each other with atomic mass of 27 amu and 28 amu respectively. They both are at correct positions with increasing atomic mass.

Additional Information:
Mendeleev in his periodic table in reference to physical and chemical properties that were related to their atomic mass in a periodic way and arranged them so that groups of elements with similar properties fell into vertical columns in his table. The Modern day periodic table that we study now expanded beyond Mendeleev's initial 63 elements. Most of the current periodic tables include 108 or 109 elements.

Note: We should note that in the modified Mendeleev's periodic table, 8 groups and 7 periods are present, whereas in modern periodic table, 18 groups and 7 periods are present, respectively. The elements which were undiscovered and for whom Mendeleev had left vacant places were discovered later. Some of them are Scandium, Gallium and Germanium. All the noble gases were known by the end of the nineteenth century and placed in separate groups.
.