Question

What is the atomic number of the last number of the seventh period of the extended form of periodic table?
(A) $116$
(B) $118$
(C) $120$
(D) $122$

Answer 1

Extended periodic table contains elements that were not composed in the periodic table earlier. This completes the seventh period. The elements above the seventh period that is in the eighth period are still hypothetical.

Complete step by step answer
The extended periodic table includes those chemical elements beyond the elements which were currently known in the periodic table. This completes the seventh row in the periodic table.
The last element of the sixth period was Redon. Its atomic number is $86$ .In seventh period, $5f,{{ }}6d,{{ }}7s,{{ }}7p$ energy subshells are completely filled. That means the number of electrons that need to be accommodated are $2 + 14 + 10 + 6 = 32$ .This infers that in the seventh period, $32$ chemical elements will be present. Now we can say that the atomic number of the last element of the extended periodic table will be $86 + 32 = 118$ . The name of this element is oganesson.
Hence, considering the above explanation the correct answer is option (B).
Additional Information:
Organesson having an atomic number of $118$ is a radioactive element. It is classified as a non-metal element and assumed to be a gas. This element is a member of the noble gas group. It is previously known as ununoctium and also in Latin it is abbreviated as one-one-eight. It still doesn’t have practical uses in real life except in scientific studies. This element is expensive also. The Organesson symbol is $Og$ .

Note
The organesson compound is made by humans. This compound is artificially made up in the lab. There is an IUPAC naming for these compounds having atomic numbers greater than $100$ . But the permanent name of this element is oganesson.