Valence (combining capacity) shows periodicity: H(1), Li(1), Be(2), B(3), C(4), N(3,5), O(2), F(1). For main group elements, max valence = group number (or 8 - group for non-metals). Variable oxidation states are common in d-block: electrons from both (n-1)d and ns can participate. Max OS usually = group number up to Mn(+7). After Mn, max OS decreases (d electrons become harder to remove due to increasing Zeff). Periodicity of oxide types: Along Period 3: Na2O (basic) → MgO (basic) → Al2O3 (amphoteric) → SiO2 (weakly acidic) → P4O10 (acidic) → SO3 (strongly acidic) → Cl2O7 (most acidic). This reflects the transition from metallic to non-metallic character.