Question

multiple-choice question
why does atomic radius increase as you go down a group?
because you add energy levels (shells)
because of shielding of the nucleus
because the nuclear charge increases

Explanation:

To determine why atomic radius increases down a group, we analyze each option:

• Option 1: "Because you add energy levels (shells)". As you move down a group in the periodic table, each element has one more electron shell (energy level) than the one above it. More shells mean the outermost electrons are further from the nucleus, increasing the atomic radius. This matches the concept.
• Option 2: Shielding (electrons in inner shells repelling outer electrons) affects effective nuclear charge but is not the primary reason for radius increase down a group (shielding is relatively constant within a group in terms of the trend's driver for radius).
• Option 3: Nuclear charge (protons in nucleus) increases down a group, but this would tend to pull electrons closer, which is the opposite of increasing radius. So nuclear charge increase is not the reason for radius increase down a group.

Answer:

A. Because you add energy levels (shells)