Which statement best describes the trend in atomic radius as you move from left to right across a period in the periodic table, and what is the main cause of this trend?

• A. Atomic radius decreases due to increasing nuclear charge, which pulls electrons closer to the nucleus.
• B. Atomic radius decreases because electrons are lost from the outer shell.
• C. Atomic radius increases because additional electron shells are added.
• D. Atomic radius remains constant because the number of protons and electrons increase equally.

Answer: (A)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured as the distance from the nucleus to the outermost electron cloud.
2. Recognize that as you move from left to right across a period in the periodic table, electrons are added to the same principal energy level (same shell), so the number of electron shells does not increase.
3. Note that protons are also added to the nucleus as you move across a period, increasing the nuclear charge (positive charge of the nucleus).
4. Because the added electrons are in the same shell, the increased nuclear charge pulls the electron cloud closer to the nucleus, reducing the atomic radius.
5. Conclude that the main cause of the decrease in atomic radius across a period is the increasing nuclear charge, which pulls electrons closer, rather than loss of electrons or addition of shells.