Question

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

Explanation:

• Shielding effect depends on the number of inner - shell electrons.
• Across a period, electrons are added to the same principal energy level.
• The number of inner - shell electrons (which cause shielding) does not change as we move across a period (since we are not adding electrons to inner shells, but to the same outer shell).

Answer:

Electrons are added to the same principal energy level.