chem review

Effective nuclear charge

The net positive charge felt by an electron, after accounting for shielding by inner electrons.

Shielding effect

Inner electrons block outer electrons from the full pull of the nucleus, reducing the effective nuclear charge.

Ionization energy (IE)

Energy needed to remove an electron from a gaseous atom. Higher IE = harder to remove an electron.

Atomic radius

The size of an atom, measured as half the distance between two bonded nuclei.

Ionic radius

The size of an ion. Cations (+) are smaller than their parent atom; anions (−) are larger.

Electronegativity (EN)

An atom’s ability to attract electrons in a bond. Highest in fluorine (EN = 4.0).

How does ionization energy (IE) change across a period (left to right)?

Increases because: More protons = stronger nuclear pull. Electrons are added to the same shell (no extra shielding).

How does ionization energy (IE) change down a group?

Decreases because: More electron shells = more shielding. Outer electrons are farther from the nucleus.

How does atomic radius change across a period?

Decreases because: More protons pull electrons closer. No new shells are added.

How does atomic radius change down a group?

Increases because: More electron shells = larger atom.

Why are cations smaller than their parent atoms?

Lost electrons = less repulsion + same nuclear pull 'shrinks' the ion.

Why are anions larger than their parent atoms?

Gained electrons = more repulsion + same nuclear pull 'expands' the ion.

How does electronegativity (EN) change across a period?

Increases because: Stronger nuclear pull attracts electrons better.

How does electronegativity (EN) change down a group?

Decreases because: More shielding + farther electrons = weaker attraction.

Which has a larger atomic radius: Na or K? Why?

K (potassium)—it has an extra electron shell compared to Na.

Which has higher IE: O or S? Why?

O (oxygen)—fewer electron shells = less shielding = harder to remove an electron.

Why is fluorine the most electronegative element?

Small size + strong nuclear pull (many protons, few shells).