Question

Anomalous pair in Mendeleev's periodic table is:

• A. $\mathrm{Li},\mathrm{Na}$
• B. $\mathrm{Co},\mathrm{Ni}$
• C. $\mathrm{Mg},\mathrm{Al}$
• D. $\mathrm{Be},\mathrm{B}$

Answer 1

The correct option is B

$\mathrm{Co},\mathrm{Ni}$

The explanation for the correct option:

(b) In the periodic table given by Mendeleev, elements were arranged in the order of their increasing atomic weights i.e.; the element with lower atomic weight came first than the element with higher atomic weight.

• But in his periodic table, some elements violated this arrangement.
• Example: Cobalt $\left(\mathrm{Co}\right)$ and Nickel $\left(\mathrm{Ni}\right)$
• Although the atomic weight of Cobalt is 58.9 u, it was placed before Nickel with an atomic weight of 58.7 u and no logic was given behind such arrangement.

The explanation for the incorrect options:

(a) Lithium $\left(\mathrm{Li}\right)$ and Sodium $\left(\mathrm{Na}\right)$ were arranged in the increasing order of their atomic weights

(c) Magnesium $\left(\mathrm{Mg}\right)$and Aluminium $\left(\mathrm{Al}\right)$ followed the order of increasing atomic weights.

(d) Beryllium $\left(\mathrm{Be}\right)$ and Boron $\left(\mathrm{B}\right)$ also followed the order of increasing atomic weights.

Hence, option (B) is correct.