Topic overview

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The periodic law states that elemental properties recur with increasing atomic number, reflecting recurring electron configurations. Across a period, effective nuclear charge generally rises, helping explain why atomic radius decreases and ionization energy and electronegativity tend to increase. Down a group, added electron shells and shielding generally correspond to larger atomic radii and lower ionization energies. Ionic size also depends on charge and electron configuration: cations are smaller than their parent atoms, anions are larger, and higher nuclear charge means a smaller radius within an isoelectronic series. The topic also introduces electron affinity, metallic and non-metallic character, reactivity, oxidation states, shielding, lanthanide contraction, size anomalies, block placement, and diagonal relationships.