Question

Numbering of groups as $1,2.........18$ was adopted by IUPAC in
A. $1986$
B. $1906$
C. $1908$
D. $1988$

Answer 1

The numbering of groups in the form of Arabic numerals is the currently adopted system of nomenclature, and was introduced in the very late $1980's$. As a result of this, the first published periodic table with the new numbering system came about two years later, in $1990$.

Complete step by step answer:
There were earlier two methods of naming groups, based on the two independent bodies, the Chemical Abstraction Service (CAS) and the International Union for Pure and Applied Chemistry (IUPAC). Let us take a look at these conventions:
The IUPAC system which was followed earlier, labelled the groups with a particular name, consisting of a Roman numeral followed by either of the letters A or B.
As a result, they were name as:
Columns $1 - 7$ were symbolized by the characters $IA - VIIA$, columns $8 - 10$ were labelled as $VIIIA$, columns $11 - 17$ were given the characters $IB - VIIB$ and lastly, the noble gases, that is, column $18$ was numbered as $VIII$.
Now coming to the CAS system,
The CAS system was also similar and only had minor differences from the old IUPAC system.
However, as we can see, this system is hard for the common man to read, understand, analyse and memorize. Thus, IUPAC proposed a new system in which the groups were numbered according to the number of electrons in their valence shell. This system was brought about in the year $1988$.

So, the correct answer is Option D .

Note: All elements of a group will have the same number of electrons in their valence shell. This is also the reason why they show similar chemical characteristics. The periods in the periodic table are numbered according to the principal quantum number of the valence shell of the atom. The new numbering system made it easier to write the electronic configurations of individual elements.