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PERIODICITY
REACTIONS OF PERIOD 3 ELEMENTS
The Period 3 elements are:
sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon
1. WITH WATER
Only sodium, magnesium and chlorine react with water under normal conditions.
Sodium
Sodium reacts violently with cold water. A piece of sodium dropped into water floats and
immediately melts into a sphere. It fizzes, giving off hydrogen, and moves over the
surface, gradually decreasing in size, until it dissolves completely. If the piece of sodium
is large enough, the hydrogen will catch fire; it burns with a bright yellow flame (due to
the presence of sodium vapour). The solution of sodium hydroxide remaining at the end
of the reaction is colourless and alkaline.
2Na(s) + 2H2O(l)
2Na+(aq) + 2OH-(aq) + H2(g)
Magnesium
Magnesium reacts very slowly with cold water. Tiny bubbles of hydrogen form on the
surface, and the metal slowly tarnishes.
Mg(s) + 2H2O(l)
Mg(OH)2(s) + H2(g)
If magnesium is heated in steam, it burns with a bright white light, releasing hydrogen
and forming a white solid (magnesium oxide).
Mg(s) + H2O(g)
MgO(s) + H2(g)
2. WITH OXYGEN
All the Period 3 elements except chlorine and argon burn in air or oxygen when ignited.
The reactions are exothermic.
Sodium
When heated, sodium melts then burns quickly with a bright yellow flame. The product
is sodium oxide, a white solid.
2Na(s) + 1/2O2(g)
Na2O(s)
Magnesium
If magnesium is heated, it burns with a bright white light, forming white smoke and a
residual white solid (magnesium oxide).
2Mg(s) + O2(g)
2MgO(s)
TOPIC 13.22: PERIODICITY
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Aluminium
Aluminium in lump form is difficult to ignite, but when aluminium powder is sprinkled into
a flame, it burns with a bright white light, forming white smoke and a residual white solid
(aluminium oxide).
2Al(s) + 3/2O2(g)
Al2O3(s)
Silicon
Silicon in lump form is difficult to ignite, but when grey silicon powder is sprinkled into a
flame, it burns with a bright white light, forming white smoke and a residual white solid
(silicon dioxide).
Si(s) + O2(g)
SiO2(s)
Phosphorus
White phosphorus is stored under water, because otherwise it could burst into flames
spontaneously without heating. The usual way of igniting phosphorus is to touch it with
a warm glass rod. It burns with a very bright white light, forming clouds of white smoke
(phosphorus(V) oxide).
P4(s) + 5O2(g)
P4O10(s)
Sulphur
Sulphur burns slowly with a blue flame; the colour is much more pronounced when it is
burnt in pure oxygen. This reaction is much less vigorous than the ones above. The
product is a colourless gas with a pungent, choking odour (sulphur(IV) oxide.
S(s) + O2(g)
SO2(g)
PROPERTIES OF THE OXIDES OF PERIOD 3 ELEMENTS
Physical Properties
Property
m.p. /K
Na2O
1548
MgO
3125
Bonding
Ionic
Ionic
Structure
Lattice
Lattice
Al2O3
2345
Ioniccovalent
Lattice
SiO2
1883
P4O10
573
SO2
200
Covalent
Covalent
Covalent
Macromolecular
Molecular
Molecular
The ionic lattices, Na2O and MgO, have strong interionic forces in three dimensions and
have high melting points.
Silicon dioxide, which is macromolecular, has strong covalent bonding in three
dimensions and has a high melting point.
Aluminium oxide, which has an intermediate ionic-covalent lattice, has strong bonds in
three dimensions and has a high melting point.
The two molecular substances, P4O10 and SO2, have only weak intermolecular forces
(dipole-dipole) and therefore have low melting points.
TOPIC 13.22: PERIODICITY
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Chemical Properties
Sodium oxide
When added to water, sodium oxide dissolves to form an aqueous solution containing
sodium hydroxide. The solution is, therefore, alkaline and has a pH of 14.
Na2O(s) + H2O(l)
2Na+(aq) + 2OH-(aq)
Magnesium oxide
Magnesium oxide is sparingly soluble in water, but when it is added to water, enough
dissolves to form a weakly alkaline solution. The pH is about 9.
MgO(s) + H2O(l)
Mg2+(aq) + 2OH-(aq)
Aluminium oxide
Aluminium oxide is insoluble in water and does not react with water. When it is added to
water, the resultant pH is 7.
Silicon dioxide
Silicon dioxide is insoluble in water and does not react with water. When it is added to
water, the resultant pH is 7.
Phosphorus(V) oxide
Phosphorus(V) oxide is very soluble in water and reacts violently with it. The product is
phosphoric(V) acid, H3PO4, a strong acid. The pH of the resultant solution is about 0.
P4O10(s) + 6H2O(l)
4H3PO4(aq)
Sulphur(IV) oxide
Sulphur(IV) oxide is fairly soluble in water and dissolves to form sulphuric(IV) acid (also
called sulphurous acid), H2SO3, a weak acid. The pH of the solution is about 3.
SO2(g) + H2O(l)
H2SO3(aq)
The properties of the oxides change across the period. The ionic metal oxides on the
left hand side are basic. The non-metal oxides on the right hand side are acidic.
Aluminium oxide, which is in the middle, is amphoteric.
Acidic oxides can react with bases to produce a salt and water.
SiO2
+
2NaOH
P4O10 + 12NaOH
TOPIC 13.22: PERIODICITY
Na2SiO3 + H2O
4Na3PO4 + 6H2O
3
Basic oxides can also react with acids to produce salt and water.
MgO + H2SO4
MgSO4 + H2O
Na2O + 2HCl
2NaCl + H2O
A substance which can react with both acids and bases is called amphoteric.
Aluminium oxide is amphoteric. It reacts with both hydrochloric acid and hot,
concentrated sodium hydroxide as shown below.
Al2O3 + 6HCl
Al2O3 + 2NaOH + 3H2O
TOPIC 13.22: PERIODICITY
2AlCl3 + 3H2O
2NaAl(OH)4
4