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

• A. The number of energy levels increases, causing the atoms to become larger.
• B. The effective nuclear charge increases, pulling electrons closer to the nucleus.
• C. The addition of more protons causes the atom to lose electrons and shrink.
• D. Electron-electron repulsion decreases, allowing the atom to expand.

Answer: (B)

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, the number of protons in the nucleus increases, which increases the nuclear charge.
3. Note that the number of energy levels (electron shells) remains the same across a period, so the electrons are being added to the same principal energy level.
4. Learn that the increased positive charge in the nucleus (effective nuclear charge) pulls the electrons closer, reducing the atomic radius.
5. Conclude that the main reason atomic radius decreases across a period is the increase in effective nuclear charge, which pulls electrons closer to the nucleus, making the atom smaller.