AN INTRODUCTION TO
PERIOD 3
COMPOUNDS
A guide for A level students
KNOCKHARDY PUBLISHING
KNOCKHARDY PUBLISHING
PERIOD 3 COMPOUNDS
INTRODUCTION
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PERIOD 3
CONTENTS
• Introduction
• Electronic configuration
• 1st Ionisation Energy
• Chlorides
• Oxides
• Reactions of metals
ELECTRONIC
CONFIGURATION
ELECTRONIC CONFIGURATION
The Aufbau principle states that… “ELECTRONS ENTER THE LOWEST
AVAILABLE ENERGY LEVEL” . In period 3 the electrons fill the 3s orbital first,
followed by the 3p orbitals. Notice how the electrons in the 3p orbitals remain
unpaired, if possible, according to Hund’s Rule.
1s
2s
2p
3s
3p
Na
1s2 2s2 2p6 3s1
Mg
1s2 2s2 2p6 3s2
Al
1s2 2s2 2p6 3s2 3p1
Si
1s2 2s2 2p6 3s2 3p2
P
1s2 2s2 2p6 3s2 3p3
S
1s2 2s2 2p6 3s2 3p4
Cl
1s2 2s2 2p6 3s2 3p5
Ar
1s2 2s2 2p6 3s2 3p6
1st IONISATION
ENERGY
FIRST IONISATION ENERGY
TREND
Theoretically, the value should increase steadily across the period
due to the increased nuclear charge. HOWEVER...
There is a DROP in the value for
aluminium because the extra
electron has gone into a 3p orbital.
The increased shielding makes the
electron easier to remove.
1500
1000
There is a DROP in the value for
sulphur. The extra electron has
paired up with one of the electrons
already in one of the 3p orbitals. The
repulsive force between the
electrons means that less energy is
required to remove one of them.
3s
3p
500
3s
Na
Mg
3p
Al
Si
P
S
Cl
Ar
CHLORIDES OF PERIOD 3 ELEMENTS
SUMMARY
NaCl
MgCl2
AlCl3
SiCl4
PCl5
melting
point / K
1074
987
450 (sub)
203
435 (sub)
bonding
ionic
ionic
covalent
covalent
covalent
structure
giant
lattice
giant
lattice
simple
molecule
simple
molecule
simple
molecule
solubility in
water
very
very
solution pH
7
6.5
hydrolysed hydrolysed
5
0
hydrolysed
0
CHLORIDES OF PERIOD 3 ELEMENTS
PREPARATION
Most can be prepared by DIRECT COMBINATION by passing the gas over
the heated element.
0
e.g.
Magnesium
Mg(s) + Cl2(g)
0
Aluminium
0
0
2Al(s) + 3Cl2(g)
+2 -1
——>
——>
MgCl2(s)
+3 -1
2AlCl3(s)
CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
NaCl
• ionic solid
+
Na
Cl
• giant ionic lattice
• high melting point – strong electrostatic attraction between ions
• very soluble in water giving a neutral (pH = 7) solution
NaCl(s)
——>
Na+(aq) + Cl¯(aq)
CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
MgCl2
• ionic solid
2+
Cl
Mg
Cl
• giant ionic lattice
• high melting point – strong electrostatic attraction between ions
• soluble in water giving a slightly acidic (pH = 6.5) solution
(due to greater charge density of Mg2+ ions)
MgCl2(s)
——>
Mg2+(aq) + Cl¯(aq)
CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
AlCl3
• white solid
• high charge density of Al3+ favours covalency
A LEWIS ACID
Cl
Al
Cl
TRIGONAL
PLANAR
Cl
• readily hydrolysed giving an acidic solution (pH = 5).
AlCl3(s) + 6H2O(l)
——>
[Al(H2O)6]3+(aq) + 3Cl¯(aq)
CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SiCl4
• simple covalent molecule
• volatile liquid
Cl
• fumes in moist air
TETRAHEDRAL
Cl
Si
Cl
Cl
• easily hydrolysed by water producing an acidic solution (pH= 0)
SiCl4(l)
+ 4H2O(l)
——> SiO2.2H2O(s) + 4HCl(aq)
CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
PCl5
• pale yellow solid
• exists as [PCl4+][PCl6¯] ; unusual for a non-metallic chloride
• hydrolysed by water giving an acidic solution (pH = 0)
PCl5(s) + 4H2O(l)
——>
H3PO4(aq) + 5HCl(aq)
OXIDES OF PERIOD 3 ELEMENTS
SUMMARY
Na2O
MgO
Al2O3
SO2
Melting
point / K
1548
3125
2345
200
bonding
ionic
ionic
ionic/cov
covalent
structure
giant
lattice
giant
lattice
giant
lattice
simple
molecule
classification
basic
basic
amphoteric
acidic
solubility
in water
very
sparingly
insoluble
reacts
14
9
solution pH
3
OXIDES OF PERIOD 3 ELEMENTS
PREPARATION
• metals produce basic oxides
• non-metals produce acidic oxides
• aluminium oxide is an amphoteric oxide (acidic and basic properties)
• ionic oxides have high melting points
• ionic oxides conduct electricity when molten
• most oxides can be prepared by direct combination (EXC. SO3)
0
0
+2 -2
2Mg(s) + O2(g) ——> 2MgO(s)
0
0
+3 -2
4Al(s) + 3O2(g) ——> 2Al2O3(s)
0
0
+4 -2
S(s) + O2(g) ——> SO2(g)
OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
Na2O
• white solid
• giant ionic lattice
• soluble in water to form a strongly alkaline solution (pH = 13-14)
Na2O(s) + H2O(l)
——> 2NaOH(aq)
• reacts with acids to form salts – A BASIC OXIDE
Na2O(s) + HCl(aq)
+
Na
——> NaCl(aq) +
2-
O
H2O(l)
+
Na
OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
MgO
• white solid
• giant ionic lattice
• sparingly soluble in water to form hydroxide (pH = 9)
(Low solubility is due to metal’s greater charge density)
MgO(s) + H2O(l)
——> Mg(OH)2(aq)
• reacts with acids to form salts – A BASIC OXIDE
MgO(s) + 2HCl(aq)
2+
Mg
——> MgCl2(aq) +
2-
O
H2O(l)
OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
Al2O3
• giant lattice with some covalent character
• insoluble in water
• AMPHOTERIC; it reacts with both acids and alkalis to give salts
——> 2AlCl3(aq) +
with acids
Al2O3(s) + 6HCl(aq)
3H2O(l)
with alkalis
Al2O3(s) + 2NaOH(aq) + 3H2O(l) ——> 2NaAl(OH)4(aq)
OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SO2
• simple covalent molecule
O
S
O
ANGULAR / BENT
• soluble in water
• reacts with water to give a weak acidic solution (pH = 3)
SO2(g) + H2O(l)
2H+(aq) + SO32-(aq)
• converted to sulphur trioxide in the CONTACT PROCESS
OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SO3
• simple covalent molecule
O
S
O
TRIGONAL
PLANAR
O
• reacts violently with water to give a strong acidic solution (pH = 0)
SO3(g) + H2O(l)
——>
2H+(aq) + SO42-(aq)
METALS OF PERIOD 3
REACTION WITH WATER
Sodium
• vigorous reaction
• hydrogen evolved
• strong alkaline solution produced (pH = 14)
Na(s) + 2H2O(l)
Magnesium
——> 2NaOH(aq) + H2(g)
• slow reaction with water
• weaker alkaline solution produced (pH 9-11)
Mg(s) + 2H2O(l)
——> Mg(OH)2(aq) + H2(g)
• very fast reaction with steam
Mg(s) + H2O(l)
——> MgO(S) + H2(g)
AN INTRODUCTION TO
PERIOD 3
COMPOUNDS
THE END
© 2007 JONATHAN HOPTON & KNOCKHARDY PUBLISHING