Why does the atomic radius decrease as electrons are added to a shell across a period in the periodic table?

• A. Because additional electrons are added to higher energy levels, which are farther from the nucleus.
• B. Because the number of protons decreases, leading to a smaller atomic radius.
• C. Because electrons repel each other more strongly, causing the atom to shrink.
• D. Because the increased nuclear charge pulls electrons closer to the nucleus, reducing the atomic radius.

Answer: (D)

Verified step by step guidance

1. Understand that as you move across a period in the periodic table, electrons are added to the same principal energy level (same shell), not to higher energy levels.
2. Recognize that with each additional proton added to the nucleus (increasing atomic number), the positive nuclear charge increases, which exerts a stronger attractive force on the electrons.
3. Note that although electrons repel each other due to their negative charges, the increasing positive charge of the nucleus has a greater effect, pulling the electron cloud closer to the nucleus.
4. This stronger attraction reduces the size of the electron cloud, causing the atomic radius to decrease across the period.
5. Therefore, the key reason for the decrease in atomic radius across a period is the increased effective nuclear charge that pulls electrons closer, despite the electrons being added to the same energy level.