Question

Assertion: Atomic size increases down the group.
Reason: since there are more filled energy levels so the valence electrons are found farther from the nucleus increasing the atomic size.
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. Both Assertion and Reason are incorrect

Answer 1

The distance between the center of the nucleus and the outermost or valence shell in an atom is called it's atomic size. The atomic size is inversely proportional to the atomic number of the elements which means that the atomic size decreases with an increase in atomic number and vice - - versa.

Complete answer:
Down the group atomic size increases but atomic size decreases across the period of moving from left to right in the periodic table.
In the period electrons are added in the same outermost shell leading to the increase in effective nuclear charge along with the nuclear charge with the increase in the atomic number because electrons shielding remains constant. Hence, the atomic size decreases across a period of moving from left to right in the periodic table.
But atomic size increases down the group in the periodic table as on going down the group the atomic radius increases as new shells get added up. In other words, the number of filled energy levels increases so the valence electrons are found farther from the nucleus leading to the increase in atomic size down the group in the periodic table.
Hence, the assertion and reason both are correct and the reason is the correct explanation of the assertion.
The correct answer is option A.

Note:
In a polyelectronic atom the net positive charge experienced by an electron is called the effective nuclear charge. Effective nuclear charge increases with an increase in atomic number thereby decreasing the atomic size. Moreover, the effective nuclear charge increases as the number of protons in an atom increases.