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

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