​Which of the following groups of elements were NOT discovered at the Mendeleev's periodic table?

Correct option is B

The correct answer is (b) Noble gases

Explanation: Mendeleev's Periodic Table is one of the most important contributions to the field of chemistry and is credited to Dmitri Mendeleev, a Russian chemist, who developed it in 1869. His periodic table was the first systematic arrangement of elements based on their atomic masses and their chemical properties.

Here's why noble gases were not included in Mendeleev's periodic table:

1. Lack of Discovery:

   • When Mendeleev created his periodic table, the noble gases (such as helium, neon, argon, krypton, xenon, and radon) had not yet been discovered. These elements were found after the development of Mendeleev's table, which is why they were absent.

2. Inertness of Noble Gases:

   • Noble gases are chemically inert because they have a full valence electron shell, making them stable and not prone to forming compounds easily. This non-reactivity made them harder to detect and study with the available experimental techniques at the time Mendeleev was working on his periodic table.

3. Later Integration of Noble Gases:

   • After the discovery of argon in 1894 by Lord Rayleigh and Sir William Ramsay, and later other noble gases like helium and neon, scientists realized that these elements should form a new group in the periodic table. This group was eventually added to the far right side of the table as Group 18 (formerly Group 0 in older systems).
   • The introduction of noble gases did not disrupt Mendeleev's periodic law but rather supported it, as these gases also exhibited periodic properties when grouped together.

4. Mendeleev’s Table and Missing Elements:

   • Mendeleev’s periodic table was arranged by atomic mass, and he left spaces for elements that were not yet discovered. This foresight allowed for the eventual inclusion of newly discovered elements, but noble gases were not part of that because they were too inert to be detected at the time.