1.7 Periodic Trends

Atomic and ionic radius trends

Atomic radius decreases left to right across a period (increasing effective nuclear charge, Zeff, pulls electrons closer) and increases down a group (more electron shells added). Cations are smaller than their parent atoms (lose outer electrons, less repulsion); anions are larger (extra electrons increase repulsion). Isoelectronic species shrink as nuclear charge (protons) increases.

Ionization energy and electronegativity

Ionization energy (IE) — energy to remove an electron — increases across a period and decreases down a group, following Zeff and radius trends. Large jumps in successive IEs reveal core electrons and confirm valence electron count. Electronegativity (attraction for shared electrons in a bond) follows the same trend: highest for fluorine, increasing up and to the right on the periodic table.

Why Zeff drives it all

Effective nuclear charge (Zeff) is the net positive charge felt by valence electrons after accounting for shielding by inner electrons. Across a period, protons increase but shielding stays roughly constant, so Zeff rises, pulling electrons in tighter—smaller radius, higher IE, higher electronegativity. Down a group, added shells increase shielding and distance, outweighing the added protons.

Key terms

Effective nuclear charge (Zeff)

The net positive charge experienced by valence electrons, accounting for shielding by inner electrons.

Atomic radius trend

Decreases across a period (left to right); increases down a group.

Ionization energy trend

Increases across a period; decreases down a group; large jumps indicate core electrons.

Electronegativity

An atom's ability to attract shared electrons in a bond; highest for fluorine, increases up and right on the table.

Isoelectronic species radius trend

For ions/atoms with the same number of electrons, radius decreases as nuclear charge (protons) increases.

Try an AP-style question

Which has the largest atomic radius?

1. Na
2. Mg
3. K
4. Ca