IB Chemistry home > Syllabus 2016 > Periodicity > The period 3 oxides

Syllabus ref: 3.2

Period 3 is the third row in the periodic table. It has eight elements beginning with sodium and ending with argon.


Oxides are compounds formed by elements with oxygen. Oxygen is a highly reactive element which reacts with most other elements to form binary compounds (compounds made up of only two elements), in which it usually has a valency of 2 minus, often with a release of large amounts of energy.

Magnesium + oxygen Magnesium oxide

Mg + O2 2MgO

Magnesium atoms transfer 2 electrons to the oxygen
Magnesium ions are created along with oxide ions

Oxides are very common in nature owing to the fact that the air consists of about 20% oxygen. Many metals occur as ores containing oxides. This makes them very important economically.


Ionic and covalent oxides

Ionic bonding occurs when there is a large difference in electronegativity between two elements. Oxygen is highly electronegative and forms ionic bonds with all metals:

sodium + oxygen sodium oxide

4Na + O2 2Na2O

Sodium oxide is a typical ionic oxide. It consists of a giant structure of sodium ions and oxide ions.

It displays the ionic character typical of a giant ionic structure, high melting point and electrical conductor when molten.

Covalent oxides are formed when oxygen reacts with non-metals:

Sulfur(IV) oxide (sulfur dioxide) is a typical simple covalent oxide. It consists of individual molecules of the formula SO2.

It has typical simple covalent properties such as low melting point and electrical electrical insulator (non-conductor) when in the liquid state.

sulfur + oxygen sulfur(IV) oxide

S + O2 SO2

Hence, there is a periodic trend from ionic oxides on the left to covalent oxides on the right.


Chemical properties of period 3 oxides

Sodium oxide

Sodium oxide, Na2O, is a white ionic compound that reacts exothermically with water producing a solution of sodium hydroxide. It is highly basic.

Na2O + H2O 2NaOH


Magnesium oxide

Magnesium oxide is a white, slightly soluble, ionic compound. It reacts with water forming magnesium hydroxide, a weak base.

MgO + H2O Mg(OH)2

Magnesium hydroxide is only slightly soluble in water.


Aluminium oxide

Aluminium oxide is a white ionic compound, which is insoluble in water. It reacts with both acids and bases forming salts. This behaviour is said to be amphoteric, i.e. it behaves as both an acid and a base, it is an amphoteric oxide.

With bases

Al2O3 + 2NaOH 2NaAlO2 + H2O

With acids

Al2O3 + 3H2SO4 Al2(SO4)3 + 3H2O


Silicon dioxide

Silicon dioxide has a giant covalent structure and is totally insoluble in water. It does react with very strong base to form salts known as silicates. This demonstrates that it is acidic oxide.

SiO2 + 2NaOH Na2SiO3 + 2H2O

silicon dioxide + sodium hydroxide sodium silicate + water


Phosphorus oxides

Both of the phosphorus oxides are acidic and react with water forming acids. phosphorus(III) oxide reacts forming phosphoric(III) acid and phosphorus(V) oxide reacts forming phosphoric(V) acid (the stronger acid of the two).

P2O3 + 3H2O 2H3PO3

phosphorus(III) oxide + water phosphoric(III) acid

P2O5 + 3H2O 2H3PO4

phosphorus(V) oxide + water phosphoric(V) acid


sulfur oxides

Both of the sulfur oxides are acidic. sulfur(IV) oxide dissolves in water forming a weak acid, whereas sulfur(VI) oxide dissolves in water forming a strong acid, sulfuric(VI) acid.

SO2 + H2O H2SO3

sulfur dioxide + water sulfuric(IV) acid

SO3 + H2O H2SO4

sulfur trioxide + water sulfuric(VI) acid

Both sulfur and phosphorus oxides demonstrate that increasing the oxidation state increases the acidity of the oxide. This is a general rule in oxides throughout the periodic table.


Chlorine oxides

Chlorine forms a series of oxides with different oxidation states, not all of which are stable. In keeping with the trends outlined above, the lower oxides are weakly acidic and the higher oxides strongly acidic.

Chlorine(I) oxide dissolves in water forming chloric(I) acid, a weak acid.

Cl2O + H2O 2HClO

Chlorine(VII) oxide dissolves in water forming chloric(VII) acid, a strong acid.

Cl2O7 + H2O 2HClO4


Summary of the period 3 oxides


P4O10 (or P4O6)

SO3 (or SO2)

Adding H2O Na2O + H2O 2NaOH MgO + H2O Mg(OH)2 Insoluble Insoluble P4O10 + 6H2O 4H3PO4 SO3 + H2O H2SO4 Cl2O7 + H2O HClO4
Adding HCl Na2O + H+ 2Na+ + H2O MgO + 2H+ Mg2+ + H2O Al2O3 + 6H+ 2Al3+ + 3H2O No reaction No reaction No reaction No reaction
Add NaOH No reaction No reaction Al2O3 + 2OH- + 3H2O 2Al(OH)4 SiO2 + 2OH- SiO32- + H2O P4O10 + 12OH- 4PO43- + 6H2O SO3 + OH- SO42- + H2O Cl2O7 + OH- 2ClO4- + H2O
Nature Basic Oxide Basic Oxide Amphoteric Oxide Acidic Oxide Acidic Oxide Acidic Oxide Acidic Oxide
Conductivity Good Good Good None None None None
mp / ºC 1275 2852 2027 1610 24 17 -92