Metallic Behaviour & Oxides
Chemistry: IB Diploma Higher Level, topic 3.2.3
Metallic and Non-Metallic Behaviour
The vertical and horizontal trends in the periodic table can be used to predict and explain the metallic and non-metallic behaviour of elements.
Metals
- Typically, metals have low ionisation energies and low electronegativity values.
- Hence the metallic character of the elements is greatest in the bottom left-hand corner of the periodic table.
Non-metals
- On the other hand, non-metals have high ionisation energies and high electronegativities.
- The most electronegative element in the periodic table is fluorine.
Covalent or ionic oxides
- The variation in metallic character on going from left to right in the periodic table is also demonstrated by the change in the type of bonding of their oxides.
- The metal oxides from Na to Al form giant ionic structures.
- The non-metal oxides from P to Cl create molecular covalent structures.
- Silicon, in the middle, is a metalloid, and silicon oxide is formed by a giant covalent structure.
Covalent or ionic oxides
- The change in the type of bonding is observed by looking at the conductivity of the compounds in the molten state.
- The metal oxides (Na to Al) conduct electricity in the molten state, whereas the non-metal oxides do not.
Acidic, Basic, and Amphoteric Oxides
Oxides change from basic through amphoteric to acidic across a period.
Basic oxides
- Metal oxides (ionic oxides) are basic and dissolve in water to produce alkaline solutions:
- Na2O(s) + H2O(l) → 2NaOH(aq)
- MgO(s) + H2O(l) → Mg(OH)2(aq)
- Basic oxides neutralise acids to produce a salt and water:
- Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l)
Acidic oxides
- Non-metal oxides (covalent oxides) are acidic and dissolve in water to give acidic solutions.
- Phosphorus (V) oxide gives phosphoric acid:
- P4O10(s) + 6H2O(l) → 4H3PO4(aq)
- Phosphorus (III) oxide gives phosphoric (III) acid:
- P4O6(s) + 6H2O(l) → 4H3PO3(aq)
- SO3 dissolves in water to give sulphuric (VI) acid:
- SO3(g) + H2O(l) → H2SO4(aq)
- SO2 dissolves in water to give sulphuric (IV) acid:
- SO2(g) + H2O(l) → H2SO3(aq)
Amphoteric oxides
- In the middle of the period, the oxides are amphoteric, meaning they can behave as both acids and bases.
- An example of an amphoteric oxide is aluminium oxide, which is insoluble in water. Aluminium oxide will neutralise both acids and bases:
- Neutralising sulphuric acid:
- Al2O3(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2O(l)
- Neutralising sodium hydroxide:
- Al2O3(s) + 3H2O(l) + 2NaOH (aq) → 2[NaAl(OH)4](aq)
Test yourself on Metallic Behaviour & Oxides
Which of these correctly describes how the nature of oxides change on going across a period?