Which factor causes the shielding effect to remain approximately constant across a period in the periodic table?

• A. Valence electrons move closer to the nucleus, increasing the shielding effect.
• B. Electrons are added to the same principal energy level, so inner-shell electron shielding does not increase.
• C. Electrons are added to higher principal energy levels, increasing the shielding effect.
• D. The number of protons decreases across a period, reducing the shielding effect.

Answer: (B)

Verified step by step guidance

1. Understand that the shielding effect refers to the reduction in effective nuclear charge on the outer electrons due to the presence of inner electrons between the nucleus and the valence electrons.
2. Recognize that as you move across a period in the periodic table, electrons are added to the same principal energy level (same shell), not to inner shells.
3. Since electrons are added to the same energy level, the inner-shell electron count remains constant, so the shielding provided by inner electrons does not increase significantly.
4. At the same time, the number of protons in the nucleus increases, which increases the nuclear charge and pulls the electrons closer, but this does not affect the shielding from inner electrons.
5. Therefore, the shielding effect remains approximately constant across a period because the inner-shell electron shielding does not increase when electrons are added to the same principal energy level.