Give reasons why “Atomic size decreases across a Period but increases down a group of the periodic table”.

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Solution

Atomic size of an atom is defined as the distance from the nucleus of that atom to its outermost shell. It is generally measured in angstroms $(A °)$ .
Atomic size decreases on moving left to right in the Period or row in the periodic table. The reason for the decrease in size is due to an increase in the effective or net nuclear charge due to the addition of electrons in the same energy shell. Due to the increase in nuclear charge felt by valence electrons, the valence electrons move towards the nucleus, and size decreases.
Example, Sodium (Na) and Chlorine (Cl) both belong to the same Period (third Period) of the periodic table. Since Sodium (Na) is comparatively present more towards the left side in the third Period, the size of Sodium (Na) is higher than Chlorine (Cl).
The number of shells and effective nuclear charge both increase on moving from top to bottom in the Group. Due to a more significant increase in the number of shells, the distance between the outermost shell and nucleus increases, and therefore, the atomic size increases from top to bottom in any Group.
Example, Fluorine (F) and Chlorine (Cl) both belong to the same Group (Group 17) but Chlorine lies below Fluorine in the Group. Therefore, the atomic size of Chlorine is higher than Fluorine.
Due to the increase in effective nuclear charge, the atomic size decreases across a period while due to the increase in the number of shells, the atomic size increases down the group.

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Q. Question 48 (a)
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(b) Electronegativity of the element decreases down the group and increases across the period
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Question 48 (c)
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Q. 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 electron shells increases.

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