Question

In the periodic table as formulated by Mendeleev, which of the following constitute an anomalous pair of elements?

• A. $H$ and $He$
• B. $Br$ and $Ne$
• C. $K$ and $Ar$
• D. $Na$ and $B$

Answer 1

The correct option is C $K$ and $Ar$

Mendeleev named as an anomalous pair for those elements whose atomic weight is less than the other element though its atomic number is more than that of the element.

e.g.- Atomic weight of $K$(39 a.m.u.) is less than atomic weight of $Ar$(40 a.m.u.).But Atomic number of $K$(19) is more than the atomic number of $Ar$(18).