Question

If a new noble gas is discovered then, what will be its atomic number?
A.$82$
B.$104$
C.$118$
D.$132$

Answer 1

We know that the last group in the periodic table is noble gas group so the last element will have completely filled configuration so its electronic configuration will be ${\text{n}}{{\text{s}}^2}{\text{,n}}{{\text{p}}^6}$.
Find the electronic configuration of each option to solve the question.

Complete step by step answer:
If a new noble gas is discovered it will be put in the seventh period and eighteenth group of the periodic table. The elements of eighteenth group of the periodic tables have fulfilled electronic configuration that’s why they are called noble gas –
As they are not reactive due to fulfilled orbitals.
So the last element will also have completely filled electronic configuration.
The noble gas elements have electronic configuration${\text{n}}{{\text{s}}^2}{\text{,n}}{{\text{p}}^6}$.
The electronic configuration of atomic number $82$ is-$\left( {{\text{Xe}}} \right){\text{4}}{{\text{f}}^{14}}{\text{5}}{{\text{d}}^{10}}{\text{6}}{{\text{s}}^2}{\text{6}}{{\text{p}}^2}$
Hence option A is incorrect.
The element having atomic number $104$ has electronic configuration-$\left( {{\text{Rn}}} \right){\text{5}}{{\text{f}}^{14}}{\text{6}}{{\text{d}}^2}{\text{7}}{{\text{s}}^2}$
Hence option C is incorrect.
The electronic configuration of last element of seventh period is $\left( {{\text{Rn}}} \right){\text{5}}{{\text{f}}^{14}}{\text{6}}{{\text{d}}^{10}}{\text{7}}{{\text{s}}^2}{\text{7}}{{\text{p}}^6}$ which is equal to atomic number $118$ as atomic number of radon (Rn) is $86$.

Hence, the correct option is ‘C’.

Note:
-The last element of the seventh period has already been found and the seventh period is complete. It is named Oganesson(Og) after scientist Oganesson.
-The noble gases are also called inert gases because they are chemically unreactive. The other characteristics of noble gases are-
-They all conduct electricity.
-They are colorless and odorless gases.
-They are used in many conditions to maintain a safe and constant environment.
-They are nonflammable gases under standard conditions.
-Noble gases glow in distinctive colors when used inside gas-discharge lamps.
-Noble gases are used in excimer lasers which are used in laser surgery, eye surgery, microlithography and micro fabrication.