Question

Assertion:In the present form of periodic table, the period number corresponds to the highest principal quantum number of the elements in the period.
Reason: Elements having similar outer configuration in their atoms belong to the same period.
A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C) Assertion is correct but Reason is incorrect.
D) Assertion and Reason are incorrect.

Answer 1

As we know that shell is the region or called principal quantum number, which is basically the collection of subshells and are labelled as K, L, M, N, etc. Also, the periodic table has rows from left to right and columns from top to bottom. Each row is called a period.

Complete solution:
As we already know that, all of the elements in a period possess the same number of atomic orbitals and these atomic orbitals also called shells are the regions corresponding to the principal quantum number and are generally labelled as K, L, M, N and so on. It is represented as $n$.
We also know that the atomic number of an element can help us to find the period number and group number because the period number is related to the number of electrons occupied shells in the element and is linked to its valence electrons.
Thus we can say that the period number corresponds to the highest principal quantum number of the element in a period. Hence the assertion is correct.
Now comes the reason, and we are very well aware of the fact that when elements possess a similar outer electronic configuration in their atoms, those elements belong to the group of the periodic table and they are arranged from top to bottom whereas the period is basically a row arranged from left to right in periodic table. Thus we can say that the reason is incorrect.

Therefore, the correct answer is (C).

Note: Always remember that a column or group or a family is placed from top to bottom and a period is placed in a row from left to right in the periodic table such as alkali metals belong to group $IA$ and alkali earth metals belong to group $IIA$ and the hydrogen and helium elements are placed in period $1$ and sodium, magnesium, aluminium etc. are placed in period $3$.