Effective Nuclear Charge & Shielding Reference Cheat Sheet

Key Facts

• Effective nuclear charge is the net positive pull felt by an electron, often estimated by $Z_{\text{eff}} \approx Z - S$.
• The atomic number $Z$ equals the number of protons in the nucleus, so a larger $Z$ means a stronger nuclear charge.
• Shielding $S$ is the reduction in nuclear attraction caused by inner electrons repelling outer electrons.
• Core electrons shield valence electrons more strongly than electrons in the same principal energy level.
• Across a period from left to right, $Z_{\text{eff}}$ generally increases because $Z$ increases while shielding changes only slightly.
• Down a group, shielding increases because atoms gain additional electron shells, so valence electrons are farther from the nucleus.
• Atomic radius usually decreases across a period as increasing $Z_{\text{eff}}$ pulls valence electrons closer to the nucleus.
• Ionization energy usually increases across a period because higher $Z_{\text{eff}}$ makes valence electrons harder to remove.

Vocabulary

Effective nuclear charge

The net positive attraction an electron feels from the nucleus after accounting for electron shielding.

Shielding

The blocking effect caused by inner electrons that reduces the attraction between the nucleus and valence electrons.

Core electrons

Electrons in inner energy levels that are not in the outermost shell and usually shield valence electrons.

Valence electrons

Electrons in the outermost occupied energy level that are involved in bonding and chemical reactivity.

Atomic radius

A measure of the size of an atom, often related to the distance from the nucleus to the valence electrons.

Ionization energy

The energy required to remove an electron from a gaseous atom or ion.