Period 3 Oxides (A2 Only)

• The first three oxides are ionic and have high melting points.
• Magnesium oxide has the highest melting point of the three.
• The electronegativity difference between aluminium and oxygen is smaller, so the aluminium ions attract the oxygen’s electrons more.
• This makes the bond partially covalent and lowers melting point.

• Silicon dioxide is a giant covalent structure and has the highest melting point of the non-metals in the third period.
• Sulfur and phosphorus oxides have molecular covalent structures.
• P₄O₁₀ has the highest melting point of the two because it has more atoms and so stronger van der Waals forces.
• SO₃ has the lowest melting point.

• Sulfur trioxide is a solid at room temp but has a low melting point (lower than the P).
• SO₂ is a gas at room temperature by contrast.

• Na₂O_(s) + H₂O_(l) → 2NaOH_(aq)
• Sodium hydroxide is a strong base, so the pH of this solution will be between 12 and 14.
• MgO_(s) + H₂O_(l) → Mg(OH)_2(aq)
• Magnesium hydroxide is a weak base, so the pH of this solution will be between 9 and 10.

The oxides of these elements do not react with water.

• P₄O_10(s) + 6H₂O_(l) → 4H₃PO_4(aq)
• This is phosphoric acid. A solution of this will have a pH of ~ 0-2.
• SO_2(g) + H₂O_(l) → H₂SO_3(aq)
• This is called sulfurous acid. It will have a pH between 0 and 2.
• SO_3(l) + H₂O_(l) → H₂SO_4(aq)
• This is sulfuric acid. It is one of the strongest acids, and will have a pH of ~ 0-2.

• Above are the structures of the sulfur oxo-acids. Both are diprotic and will lose two protons in solution.

• Above is the structure of phosphoric acid. It is triprotic, so will lose three protons in solution and form a tri-anion.