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Question
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If the seventh period is completed , the atomic number of the last element will be-
A)118
B)112
C)107
D)120

Answer Verified Verified
Hint: Since the last group is noble gas group so the last element will have completely filled configuration and it is placed with noble gases so its electronic configuration will be ${\text{n}}{{\text{s}}^2}{\text{,n}}{{\text{p}}^6}$.

Complete step by step answer:
The elements of the seventh 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. Because it will be placed in the seventh group only if its properties match with the noble gas elements. The noble gas elements have electronic configuration ${\text{n}}{{\text{s}}^2}{\text{,n}}{{\text{p}}^6}$. So 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 ‘A’.

Additional information:
 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.

Note:
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 colourless and odourless 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 microfabrication.