How does effective nuclear charge change across a period, and how does this affect atomic radius?

How does effective nuclear charge change across a period, and how does this affect atomic radius?

• A. Zeff increases across a period; atomic radius decreases.
• B. Zeff decreases across a period; atomic radius increases.
• C. Zeff remains constant; atomic radius decreases.
• D. Zeff increases across a period; atomic radius increases.

Answer: (A)

As you move across a period, the number of protons in the nucleus increases while the electrons are added to the same principal shell. The shielding from inner electrons stays roughly constant, so the increased nuclear charge is felt more strongly by the outer electrons. This higher effective nuclear charge pulls the outer electrons closer to the nucleus, reducing the atomic radius. In short, Zeff increases across a period, leading to a decrease in atomic radius.

As you move across a period, the number of protons in the nucleus increases while the electrons are added to the same principal shell. The shielding from inner electrons stays roughly constant, so the increased nuclear charge is felt more strongly by the outer electrons. This higher effective nuclear charge pulls the outer electrons closer to the nucleus, reducing the atomic radius. In short, Zeff increases across a period, leading to a decrease in atomic radius.