✔ 最佳答案
The outer electrons are screened by the inner electrons, and thus the attractive forces between the outer electrons and the nucleus are decreased. This is known as screening effect (or shielding effect). Usually, we only consider that the outermost shell electrons that are screened by the inner electrons.
The outermost shell electrons are only screened by the inner electrons, and the screening effect of outermost shell electrons to other outermost shell electrons is negligible.
The screening effect can decrease the attractive forces between the outermost shell electrons and the nucleus. In other words, the screening effect can lower the nucleus charge experienced by the outermost shell electrons. Thus,
(Effect nuclear charge) = (Nuclear charge) - (Screening effect)
Across a period in the Periodic Table, the number of shell is unchanged. For each atom of the elements in the period, the outermost shell is screened by the same number of inner electrons, and thus the screening effect is (nearly) constant. However, across the period, the nuclear charge is increasing and this leads to the increase of effective nuclear charge. Since the outermost shell electrons are experiencing an increasing effective nuclear charge across the period, the atomic radius is decreasing.
=
