Which of the following best describes the trend of atomic radius across a period and down a group in the periodic table?

• A. Atomic radius increases both across a period and down a group.
• B. Atomic radius increases across a period and decreases down a group.
• C. Atomic radius decreases across a period and increases down a group.
• D. Atomic radius decreases both across a period and down a group.

Answer: (C)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured from the nucleus to the outer boundary of the electron cloud.
2. Recall that across a period (left to right) in the periodic table, the number of protons in the nucleus increases, which increases the effective nuclear charge experienced by electrons.
3. Because the effective nuclear charge increases across a period, electrons are pulled closer to the nucleus, causing the atomic radius to decrease across a period.
4. Down a group (top to bottom), new electron shells are added, increasing the distance between the outermost electrons and the nucleus.
5. The addition of electron shells down a group outweighs the increase in nuclear charge, so the atomic radius increases as you move down a group.