Which factor is primarily responsible for the decrease in atomic radius of elements across period 3 from left to right?

• A. Increasing electron-electron repulsion in the outer shell
• B. Increasing number of electron shells
• C. Decreasing number of protons
• D. Increasing nuclear charge with constant shielding

Answer: (D)

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. Recognize that across Period 3 (from left to right), the number of protons in the nucleus increases, which means the nuclear charge increases.
3. Note that the electrons are being added to the same principal energy level (same shell), so the shielding effect remains approximately constant.
4. Because the nuclear charge increases while shielding stays constant, the effective nuclear charge experienced by the outer electrons increases, pulling the electrons closer to the nucleus.
5. Conclude that this increased attraction due to higher effective nuclear charge causes the atomic radius to decrease across Period 3.