Bonding and structure of period 3 oxides

The oxides of period 3 elements show a range of different types of bonding and structures:

• Na₂O and MgO are ionic metal oxides that form giant ionic lattice structures. This is because the large electronegativity difference between the metal and oxygen atoms creates strong ionic bonds between oppositely charged ions.
• Al₂O₃ is an ionic oxide that forms a giant ionic lattice structure with mixed bonding. The small, highly charged Al³⁺ ion can get close to the O^2- ion and distort its electron cloud, resulting in the formation of a partial covalent bond between Al and O.
• SiO₂ forms a giant covalent network with a continuous, strong covalent structure extending in all directions. Each silicon atom shares electrons with four oxygen atoms and each oxygen shares electrons with two silicon atoms.
• P₄O₁₀ and SO₃ form simple molecular structures where discrete molecules are held together by weak intermolecular forces. Within each molecule there are strong covalent P-O and S-O bonds.

Melting points relate to bonding and structure

The strength of bonds in an oxide determines the amount of energy needed to melt it from a solid to a liquid. The stronger the bonds in an oxide, the more energy is required to break those bonds and the higher its melting point.

The ionic oxides Na₂O, MgO and Al₂O₃ have high melting points because their lattice structures are held together by strong electrostatic forces between oppositely charged ions, requiring substantial energy to overcome.

• MgO has a higher melting point than Na₂O because the Mg²⁺ ion is smaller and more highly charged than the Na⁺ ion. This creates stronger electrostatic attractions between ions in the MgO lattice structure.
• Al₂O₃ has a lower melting point than MgO because the ionic bonding in Al₂O₃ has some covalent character, meaning the electrostatic forces between ions are weaker.

SiO₂ has the next highest melting point because of the many strong covalent silicon-oxygen bonds in its giant covalent structure.

The simple molecular oxides P₄O₁₀ and SO₃ have the lowest melting points as their molecules are held together by weak intermolecular forces which require the least amount of energy to overcome.

• P₄O₁₀ has a slightly higher melting point than SO₃ because it forms larger molecules with more electrons available to create stronger induced dipole-dipole forces between P₄O₁₀ molecules.

Reactions of period 3 oxides with water

The oxides show different reactions with water due to differences in their structure and bonding.

The table below summarises the bonding, structure and reactions with water for various period 3 oxides:

The general trend is that the solutions of period 3 oxides shift from alkaline to acidic moving across the period.

Sodium and magnesium oxides form alkaline solutions

Na_2_O and MgO have giant ionic lattice structures. When dissolved in water, these lattices separate into hydrated ions:

• Na₂O_(s) ➔ 2Na⁺_(aq) + O²⁻_(aq)

• MgO_(s) ➔ Mg²⁺_(aq) + O²⁻_(aq) The oxide ions (O²⁻) react with water molecules to form hydroxide ions (OH⁻):

• O²⁻_(aq) + H₂O_(l) ➔ 2OH⁻_(aq) This makes the solution alkaline. For example:

• Na₂O_(s) + H₂O_(l) ➔ 2NaOH_(aq) (pH 12-14)

• MgO_(s) + H₂O_(l) ➔ Mg(OH)_2(aq) (pH ≈10)

Aluminium oxide and silicon dioxide are insoluble

• Al₂O₃ has an ionic structure with some covalent character. It does not dissolve or react with water partly due to the additional covalent bonding it has.
• SiO₂ has a giant covalent structure, so it does not dissolve or react with water.

Phosphorus pentoxide and sulfur oxides form acidic solutions

P₄O₁₀, SO₂ and SO₃ have simple molecular structures.

When water is added, they react vigorously and dissolve to form acidic solutions containing hydrogen ions (H⁺) that are donated as the acidic oxides dissolve.

• Phosphorus pentoxide P₄O₁₀ dissolves in water to form phosphoric(V) acid: P₄O_10(s) + 6H₂O_(l) ➔ 4H₃PO_4(aq) This reaction produces an acidic solution with a pH of 2.

• Sulfur dioxide SO₂ dissolves in water to form sulfurous acid: SO_2(g) + H₂O_(l) ➔ H₂SO_3(aq)

• Sulfur trioxide SO₃ dissolves in water to form sulfuric acid: SO_3(g) + H₂O_(l) ➔ H₂SO_4(aq)

• The sulfur oxides reacting with water create acidic solutions with a pH of 0-3.