Effective Nuclear Charge (Zeff): Net positive charge experienced by valence electrons ($Z_{eff} = Z - S$). Increases across periods, causing atomic radius contraction.
Atomic & Covalent Radii: Decreases from Group 1 to Group 18 across a row due to higher $Z_{eff}$; expands down groups due to principal quantum shell additions ($n$).
First Ionization Energy: Energy required to remove the outermost electron. Exhibits peaks at noble gases and inflections at subshell completions ($s^2$, $p^3$).
Electronegativity (Pauling): Tendency of an atom in a molecule to attract shared electron pairs. Fluorine is maximum ($3.98$), Francium/Cesium minimum ($0.7$).
Crystal Lattice Types: Solid state atomic geometries including Face-Centered Cubic (FCC, 74% APF), Body-Centered Cubic (BCC, 68% APF), and Hexagonal Close-Packed (HCP).
Isotope Systematics: Stable vs radioactive nuclides governed by the Valley of Stability and N/Z ratios.