Question

ASSERTION-First period of periodic table is the shortest period
REASON-K shell can accommodate a minimum of two electrons
(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 is incorrect but reason is correct.

Answer 1

Hint: There are seven periods in total in the periodic table. The number of any period corresponds to the valence shell of the elements in it.

Complete answer:
A period in a periodic table is the horizontal row in which the elements are arranged in increasing order of the atomic number. Once the electrons are completely filled in an orbital the period ends and the succeeding period starts with electrons entering a new shell.
First period consists of two elements hydrogen (Z=1) and helium (Z=2). Second and third periods have eight elements followed by fourth and fifth with 18 elements each. Sixth and seventh periods are made of 32 elements each. From the first period the electrons start from entering the K-shell and in the seventh period the valence electrons fill up the N-shell.
The number of electrons a shell can accommodate can be found from the formula ${\text{2}}{{\text{n}}^{\text{2}}}$ where n for K-shell is 1. Substituting the value of n=1 we get ${\text{2}}{{\text{n}}^{\text{2}}}{\text{ = 2}}$, as elements are filled in the period in increasing order of atomic number by one unit, the first period can accommodate only two elements. So, the first period containing only two elements is the shortest period.

The answer for the above question is option (A).

Note:
K, L, M, N are the names given to the energy levels into which electrons in a given element are found moving around the nucleus. The names are given by the spectroscopic study of x-rays of all elements.