Question

what causes the shielding effect to remain constant across a period? electrons are added to different principal energy levels. the charge on the nucleus is constant. electrons are added to the same principal energy level. the atomic radius increases.

Explanation:

The shielding effect is related to the number of inner - shell electrons. Across a period, electrons are added to the same principal energy level. Since the number of inner - shell electrons (which are responsible for shielding) does not change (as we are not moving to a new principal energy level for the inner electrons), the shielding effect remains relatively constant.

• Option 1: If electrons were added to different principal energy levels (like when moving down a group), the shielding effect would change more significantly.
• Option 2: The charge on the nucleus (atomic number) increases across a period.
• Option 4: The atomic radius generally decreases across a period (due to increased nuclear charge attracting electrons more strongly when shielding is relatively constant).

Answer:

Electrons are added to the same principal energy level.