Question

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

Explanation:

• Shielding effect is related to the number of inner - shell electrons.
• Across a period (left to right in the periodic table), electrons are added to the same principal energy level.
• The number of inner - shell electrons (which cause shielding) does not change across a period as electrons are added to the outermost (same principal) energy level.
• The nuclear charge increases across a period (not constant), atomic radius decreases (not increases) across a period. And if electrons were added to different principal energy levels (which is the case down a group, not across a period), shielding would change.

Answer:

Electrons are added to the same principal energy level.