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Assertion: atomic size decreases across period but increases down a group of the periodic table.
Reason: moving from left to right across a period, electrons are added one at a time to the outer energy shell. Electrons within a shell cannot shield each other from the attraction to protons, while moving down a group in the periodic table, the number of electrons and filled electrons shells increases.
A. both assertion and reason are incorrect 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

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Last updated date: 24th Jun 2024
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Answer
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Hint:. As we know that atomic radius is the measure of the size of the atom. It is basically the mean distance from the centre of the nucleus to the surrounding shells of electrons. It is found that atomic radius shows different trends across a period and down the group in the periodic table.

Complete step by step answer:
- It is found that in the same period, electrons are filled in the same shell. Hence, they are not able to screen each other. Due to this it is found that the nuclear charge and the effective nuclear charge increases. And hence the radius decreases across the period.
- We can see that in the group, new shells are being added and due to this the radius is found to increase down a group of the periodic table.
- Hence, we can conclude that the correct option is (a), that is both assertion and reason are incorrect and reason is the correct explanation for assertion.
So, the correct answer is “Option A”.

Note: - As we know that Van der Waal radius is the measure of the size of the atom that is not chemically bonded. It is defined as half of the closest distance of two equal as well as non-covalently bonded atoms in a molecule.