Question

The Russian chemist Dmitri Mendeleev is generally credited with organizing the chemical elements into a comprehensive periodic table in large part due to the fact that he was able to use his table to predict the existence of elements, like gallium, that were not known at the time (1872), but were discovered later. Which is the best explanation for why Mendeleev was unable to predict the existence of the noble gases (group 18)?
A.Noble gases are not found on Earth.
B.Noble gases are stable and very unreactive, and therefore, have few observable properties.
C. Noble gases are synthetic elements and do not occur naturally.
D.Noble gases are very reactive and do not exist as stable elements.

Answer 1

We will discuss the Mendeleev’s Periodic Table and the merits and demerits of this periodic table. All the Elements in the periodic table were arranged in the increasing order of their relative atomic masses.

Complete step by step answer: Noble gases were discovered much later after Mendeleev. After the invention of inert gas, they were placed during a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements within the Mendeleev’s table. Noble gases are chemically unreactive and present in very low concentration within the atmosphere. Mendeleev left some blank spaces in his table so as to put the substances having similar properties within the same group. For example; titanium has been placed in the 4th group, leaving a space adjacent thereto in 3rd group.
Hence option B is correct one.

Additional information: There were eight groups and seven periods in the Mendeleev periodic table.
According to Mendeleev, normal elements were inserted in between group I to VII while group VIII were basically for transition elements.
There were two subgroups in which group I to VII were divided while on the other hand group VIII only comprised three elements.
All the elements in periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
Most of the elements showing similar properties were arranged in the same group like lithium, potassium, rubidium, etc. were in the 1st group.

Note: Position of Hydrogen: Hydrogen has been placed in 1st group with alkali metals, since hydrogen makes compounds within the same way as alkali metals do.
- On the other hand, hydrogen exists as a diatomic molecule; almost like halogen and hydrogen makes covalent compounds also as halogens do.