Why does atomic radius decrease as you move from left to right across a period in the periodic table?

• A. Because the number of neutrons increases, causing the atom to shrink.
• B. Because the number of protons increases, resulting in a greater effective nuclear charge that pulls electrons closer to the nucleus.
• C. Because additional electron shells are added, increasing the distance between the nucleus and outer electrons.
• D. Because electrons are lost from the outer shell, reducing the size of the atom.

Answer: (B)

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, the number of protons in the nucleus increases. This increase in positive charge is called the nuclear charge.
3. Note that while electrons are also added across the period, they are added to the same principal energy level (same shell), so the shielding effect does not increase significantly.
4. Because the effective nuclear charge (Z_eff) felt by the outer electrons increases, the nucleus pulls the electrons closer, reducing the atomic radius.
5. Conclude that the increase in nuclear charge without a significant increase in shielding causes the atomic radius to decrease across a period.