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 (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. 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)

• The pH changes for the reactions of the oxides with water can be explained by reference to the following equations

Reaction of Oxides with Water

• The pattern here is that:
  • The metallic oxides form hydroxides when they react with water
  • The non-metallic oxides form oxoacids when they react with water

• The position of an element in the periodic table can be used to predict and explain its?metallic?and?non-metallic?behaviour
• This is illustrated by the bonding of the oxides
• Metal and non-metal elements generally form?ionic?compounds so the elements?Na?to?Al?have?giant ionic structures
• The oxides become more?ionic?as you go?down the group?as the?electronegativity decreases
• The oxides become less?ionic?as you go?across a period?as the?electronegativity increases
• The oxides of non-metals such as?S,?N?and?P?form?molecular covalent compounds

• Sometimes you may be asked to make predictions about oxides that are not specifically mentioned in the syllabus but you should be able deduce their properties if you understand the patterns outlined above, as the following example shows:

Answer:

The correct option is?D.

• • CO_2?and SO_3?will produce a pH below 7 as they are non-metal oxides. CaO and Na₂O will produce a pH above 7 as they are metal oxides; however the pH is decreasing as you go across a period, so Na₂O will have a higher pH than CaO