Periodic Properties & Trends

Z_eff = protons minus shielding by core and same-shell electrons (core shields well, same-shell poorly). It rises left-to-right across a period and drives the radius and IE trends.

Radius increases down a group (higher n shell) and decreases across a period (higher Z_eff pulls the same-shell valence electrons in tighter).

IE₁ rises across a period, falls down a group. Two dips: Be → B (B loses a higher-energy 2p) and N → O (O loses a paired 2p, restoring half-filled stability).

IE₁ < IE₂ < IE₃ … always. A sharp jump appears when the next electron removed is a core electron: a jump after IEₙ means n valence electrons.

EA generally becomes more negative across a period. Exceptions: groups 2, 15, 18. Cl has the most negative EA (adding an electron to tiny F is destabilized by repulsion).

Electronegativity (Pauling) increases up and to the right; F is highest (χ = 3.98). Use it qualitatively, for ranking and comparison.

A cation is smaller than its atom (fewer electrons, higher Z_eff each; main-group cations expose the inner shell).

An anion is larger than its atom (added electrons raise repulsion, lower Z_eff each). S: 104 → S²⁻ 170 pm.

Same electron count = isoelectronic. Radius shrinks as Z rises. 10-e⁻ set: N³⁻ > O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺ > Al³⁺.

Same-group elements share a valence configuration (Ca, Sr, Ba are ns²) so they share reactivity, oxidation states, and bonding. Diagonal likenesses (Be↔Al, Li↔Mg) are secondary.