Oxides
Oxides across a period
- The acid-base character of the oxides provides evidence of chemical trends in the periodic table
- The broad trend is that oxides change from basic through amphoteric to acidic across a period
- Aluminium oxide is amphoteric which means that it can act both as a base (and react with an acid such as HCl) and an acid (and react with a base such as NaOH)
Acidic & Basic Nature of the Period 3 Oxides
- The acidic and basic nature of the Period 3 elements can be explained by looking at their structure, bonding and the Period 3 elements’ electronegativity
Structure, Bonding & Electronegativity of the Period 3 Elements Table
- The difference in electronegativity between oxygen and Na, Mg and Al is the largest
- Electrons will therefore be transferred to oxygen when forming oxides giving the oxide an ionic bond
- The oxides of Si, P and S will share the electrons with the oxygen to form covalently bonded oxides
- The oxides of Na and Mg which show purely ionic bonding produce alkaline solutions with water as their oxide ions (O2-) become hydroxide ions (OH-):
O2-(aq) + H2O(l) → 2OH-(aq)
- The oxides of P and S which show purely covalent bonding produce acidic solutions with water because when these oxides react with water, they form an acid which donates H+ ions to water
- Eg. SO3 reacts with water as follows:
SO3(g) + H2O(l) → H2SO4(aq)
- The H2SO4 is an acid which will donate an H+ to water:
H2SO4(aq) + H2O(l) → H3O+ (aq) + HSO4-(aq)
Examiner Tip
Only examples of general trends across periods and groups are required, but you should be able to link trends in ionization energy, electron affinity and electronegativity with trends in chemical character such as the nature of the oxides and metallic/ non-metallic behaviour.