4 Acidic and basic character in the periodic table

• Across a period: as atomic number increases the basic character of the oxide decreases and the acidic character increases. Period 3: Na₂O (strongly basic), MgO (basic), Al₂O₃ (amphoteric), SiO₂ (weakly acidic), P₄O₁₀ (acidic), SO₃ (strongly acidic), Cl₂O₇ (very strongly acidic).
  • P₄O₁₀ + 6H₂O → 4H₃PO₄; Cl₂O₇ + H₂O → 2HClO₄.
• Down a group (e.g. group 1): basic character of the hydroxides/oxides increases as atomic number increases (LiOH < NaOH < KOH < RbOH < CsOH).
• For the hydrides of group 7A the acidic strength increases down the group: HF < HCl < HBr < HI. As the radius of the halogen increases, the attraction of the halogen to the hydrogen atom decreases, so hydrogen is ionised more easily.

Worked example 4.3. Write balanced equations showing that sodium oxide is basic and sulphur trioxide is acidic. Na₂O + 2HCl → 2NaCl + H₂O; SO₃ + 2NaOH → Na₂SO₄ + H₂O.

Worked example 4.4. Show that tin(II) oxide is amphoteric. SnO + 2HCl → SnCl₂ + H₂O; SnO + 2NaOH → Na₂SnO₂ + H₂O.

Worked example 4.5. Classify the oxides CaO, SO₂, ZnO, NO, Li₂O. CaO basic; SO₂ acidic; ZnO amphoteric; NO neutral; Li₂O basic.

Worked example 4.6. Which is more acidic, HI or HF in water? HI, since the H–I bond is longer and weaker.

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