: 200
Metallic character (tendency to lose electrons) decreases across a period and increases down a group. Most metallic: Cs/Fr. Most non-metallic: F. Metalloids (B, Si, Ge, As, Sb, Te) lie along the staircase boundary — they are semiconductors.
Oxide character follows metallic trend: Across Period 3: Na2O (strongly basic) → MgO (basic) → Al2O3 (amphoteric) → SiO2 (weakly acidic) → P4O10 (acidic) → SO3 (strongly acidic) → Cl2O7 (most acidic).
Down a group: oxides become more basic. Group 15: N2O5 (acidic) → P4O10 (acidic) → As2O3 (amphoteric) → Bi2O3 (basic). This reflects increasing metallic character downward.
Higher oxidation state oxides are more acidic: CrO (basic) vs CrO3 (acidic); MnO (basic) vs Mn2O7 (acidic).