The oxides formed by the Period 3 elements are Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3. Their bonding and structure change across the period in the same way as the elements do.Giant ionic lattices: Na2O, MgO and Al2O3. The metal ions and oxide ions (O2−) are held in a lattice by strong electrostatic attraction. Aluminium oxide is ionic but has some covalent character: the small, highly charged Al3+ ion polarises (distorts) the electron cloud of the oxide ion.Macromolecular: SiO2. Each silicon atom is covalently bonded to four oxygen atoms and each oxygen to two silicon atoms, in a giant covalent network.Simple molecular: P4O10, SO2 and SO3. Strong covalent bonds hold the atoms together inside each molecule, but only weak intermolecular forces act between molecules.
Why the change? Oxygen is very electronegative. Sodium and magnesium have low electronegativity, so the electronegativity difference with oxygen is large and electrons are transferred (ionic bonding). Across the period the elements become more electronegative, the difference shrinks and the electrons are shared instead (covalent bonding).Worked example: find the oxidation state of sulfur in SO3. Each O is −2, so 3 × (−2) = −6, and S must be +6.
Why the change? Oxygen is very electronegative. Sodium and magnesium have low electronegativity, so the electronegativity difference with oxygen is large and electrons are transferred (ionic bonding). Across the period the elements become more electronegative, the difference shrinks and the electrons are shared instead (covalent bonding).Worked example: find the oxidation state of sulfur in SO3. Each O is −2, so 3 × (−2) = −6, and S must be +6.