Period 3 oxides

• 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

Reactions of the Period 3 oxides with acid/base table

• 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 binding
• The oxides of Si, P and S will?share?the electrons with the oxygen to form?covalently bonded?oxides
• The?giant ionic?and?giant covalent?structured oxides will have?high melting points?as it is difficult to break the structures apart

The diagram shows the structure of some Period 3 oxides

• The oxides of?Na and Mg?which show purely?ionic bonding?produce?alkaline?solutions with water as their?oxide?ions (O^2-) become?hydroxide?ions (OH^-):

O^2-(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. SO_3?reacts with water as follows:

SO₃(g) + H₂O(l) → H₂SO₄(aq)

• • The H₂SO₄?is an acid which will donate a H⁺?to water:

H₂SO₄(aq) + H₂O(l) → H₃O⁺?(aq) + HSO₄^-(aq)

• Al?and?Si?are insoluble and when they react with?hot, concentrated alkaline solution?they act as a base and form a salt
  • This behaviour is very typical of a?covalently bonded oxide
• Al?can also react with?acidic solutions?to form a salt and water
  • This behaviour is very typical of an?ionic bonded metal oxide
• This behaviour of?Al?proves that the chemical bonding in aluminium oxide is not purely?ionic?nor?covalent: it is amphoteric

Period 3 hydroxide

• NaOH is a strong base and will react with acids to form a salt and water:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

• Mg(OH)₂?is also a basic compound which is often used in indigestion remedies by neutralising the excess acid in the stomach to relieve pain:

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

• Al(OH)₃?is?amphoteric?and can acts both as an acid and base:

Al(OH)₃(s) + 3HCl(aq) → AlCl₃(s) + 3H₂O(l)

Al(OH)₃(s) + NaOH(aq) → NaAl(OH)₄(aq)