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 depends on the number of inner - shell electrons. Across a period (horizontal row in the periodic table), 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 electrons are added to the outermost shell of the same principal energy level), the shielding effect remains relatively constant.

• Option 1: If electrons were added to different principal energy levels (as in moving down a group), the shielding effect would change.
• Option 2: The charge on the nucleus (atomic number) increases across a period.
• Option 4: Atomic radius decreasing (not increasing, due to increased nuclear charge attracting electrons more strongly) is a result of other factors related to the increase in nuclear charge and relatively constant shielding, not the cause of constant shielding.

Answer:

Electrons are added to the same principal energy level.