3.5.2 Periodicity - reactions of Period 3 elements Na to Ar to illustrate periodic trends


Students should:
  • be able to describe trends in the reactions of the elements with water, limited to Na and Mg
  • be able to describe the trends in the reactions of the elements Na, Mg, Al, Si, P and S with oxygen, limited to the formation of Na2O, MgO, Al2O3, SiO2, P4O10 and SO2.


Reaction with oxygen

Sodium is a highly electropositive element, which is reactive and burns easily in air or oxygen forming the oxide.

4Na + O2 2Na2O

Its oxide, sodium oxide, Na2O, is a white powder with a giant ionic structure. In keeping with giant ionic structures, it has a high melting and boiling point. When molten it is a good conductor of electricity, typical of an ionic compound.

Reaction with water

Sodium reacts vigorously with water producing sodium hydroxide solution and hydrogen gas. The exothermic reaction is accompanied by a great deal of heat energy and the sodium always melts and often ignites with a yellow flame, setting fire to the hydrogen in the process.

Na(s) + H2O(l) NaOH(aq) + H2(g)

Reactions of sodium


Reaction with oxygen

Magnesium is an electropositive reactive metal, which burns easily in air or oxygen forming magnesium oxide, MgO.

2Mg + O2 2MgO

Magnesium oxide is a white powdery compound with a giant ionic structure. In this case the giant ionic structure has double charged magnesium ions making its lattice relatively much stronger than that of sodium oxide. When molten it is a good conductor of electricity, typical of an ionic compound.

Reaction with water

Magnesium reacts extremely slowly with water forming the sparingly soluble magnesium hydroxide and hydrogen gas. If a piece of magnesium ribbon is immersed under a filter funnel and the hydrogen collected, it will take a couple of weeks to obtain just a few cm3 of gas

Mg(s) + 2H2O(l) Mg(OH)2(aq) + 2H2(g)

However, if magnesium is heated in steam the reaction is extremely vigorous and often melts glass test tubes. Notice that the product in this case is magnesium oxide, not the hydroxide.

2Mg + H2O 2MgO + H2


Reaction with oxygen

Aluminium reacts directly with oxygen if heated to an elevated temperature or when in finely divided power form. The product is aluminium oxide:

4Al + 3O2 2Al2O3

Aluminium oxide is an pure white ionic solid with a very high melting point. Inbterestingly, it is the main constituent of rubies, crystals in which the red colour is attributable to iron impurity.


Reaction with oxygen

Silicon reacts directly with oxygen to form silicon dioxide.

Si + O2 SiO2

Silicon dioxide has a giant covalent macromolecular structure, a reflection of silicon's classification as a non-metal.


Reaction with oxygen

Phosphorus reacts directly with oxygen extremely easily - in fact white phosphorus ignites spontaneously in warm air. In the presence of excess oxygen the pentoxide of phosphorus, phosphorus(V) oxide, is formed, but in limited air or oxygen phosphorus trioxide, phosphorus(III) oxide, is formed.

P4 + 5O2 P4O10

P4 + 3O2 P4O6

Both compounds are white, simple molecular substances with low melting points.


Reaction with oxygen

sulfur is a yellow crystalline solid that burns steadily in air or oxygen with a pale blue flame forming sulfur(IV) oxide (sulfur dioxide), a choking acidic gas with a metallic smell.

Sulfur itself exists as monoclinic or rhombic crystals, containing crowns of S8 molecules. However, in equations it is usually simplified to S.

S + O2 SO2