Answer :

Final answer:

The atomic radius increases as one moves down a group in the periodic table because additional electron shells are added. Conversely, atomic radius decreases across a period due to a stronger effective nuclear charge pulling electrons closer to the nucleus.

Explanation:

The relationship between atomic radius and electron shell size is a fundamental concept in Chemistry. The atomic radius is essentially the size of an atom, often measured from the nucleus to the outer electron shell. As one moves down a group in the periodic table, the atomic radii increase due to the addition of electron shells, which increases the principal quantum number, and thus, the outermost electrons are farther from the nucleus. On the contrary, as one moves across a period, from left to right, the radii decrease because of the increased effective nuclear charge (Zeff), which pulls the electrons closer to the nucleus despite the increase in the number of electrons. This is due to the shielding effect being less significant within the same principal quantum shell. Therefore, the size of the atom and its covalent radius increase with increased electron shell size in vertical groups, and decrease across periods due to increased Zeff.

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