Which of the following best explains why the atomic radius decreases as you move from left to right across a period in the periodic table?

• A. Electrons are added to higher energy levels, increasing the size of the atom.
• B. The atoms gain more neutrons, which compresses the electron cloud.
• C. The number of electron shells increases, causing the outer electrons to be farther from the nucleus.
• D. The number of protons increases, resulting in a stronger effective nuclear charge that pulls electrons closer to the nucleus.

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 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), not to higher energy levels.
3. Note that the number of protons in the nucleus increases across a period, which increases the positive charge of the nucleus.
4. Realize that the increased positive charge (more protons) exerts a stronger attractive force on the electrons, pulling them closer to the nucleus.
5. Conclude that this stronger effective nuclear charge reduces the atomic radius, causing the atom to become smaller from left to right across a period.