1. A guide for A level students KNOCKHARDY PUBLISHING
AN INTRODUCTION TO PERIOD 3 COMPOUNDS
A guide for A level students
KNOCKHARDY PUBLISHING

2. KNOCKHARDY PUBLISHING
PERIOD 3 COMPOUNDS
INTRODUCTION
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3. PERIOD 3 CONTENTS Introduction Electronic configuration
1st Ionisation Energy
Chlorides
Oxides
Reactions of metals

4. ELECTRONIC CONFIGURATION

5. 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.
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p6 Na Mg Al Si P S Cl Ar
1s s 2p s p

6. 1st IONISATION ENERGY

7. 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.
Na Mg Al Si P S Cl Ar
1500
1000
500
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 p
3s p

8. CHLORIDES OF PERIOD 3 ELEMENTS
SUMMARY
NaCl MgCl AlCl SiCl PCl5
melting
point / K (sub) (sub)
bonding ionic ionic covalent covalent covalent
structure giant giant simple simple simple
lattice lattice molecule molecule molecule
solubility in
water very very hydrolysed hydrolysed hydrolysed
solution pH

9. CHLORIDES OF PERIOD 3 ELEMENTS
PREPARATION
Most can be prepared by DIRECT COMBINATION by passing the gas over
the heated element.
e.g. Magnesium Mg(s) Cl2(g) ——> MgCl2(s)
Aluminium 2Al(s) Cl2(g) ——> 2AlCl3(s)
+2 -1
+3 -1

10. CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
NaCl • ionic solid
• 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) Cl Na +

11. CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
MgCl2 • ionic solid
• 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) Cl Cl Mg 2+

12. CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
AlCl3 • white solid
• high charge density of Al3+ favours covalency
• readily hydrolysed giving an acidic solution (pH = 5).
AlCl3(s) H2O(l) ——> [Al(H2O)6]3+(aq) Cl¯(aq) Al Cl
A LEWIS ACID
TRIGONAL PLANAR

13. CHLORIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SiCl4 • simple covalent molecule
• volatile liquid
• fumes in moist air
• easily hydrolysed by water producing an acidic solution (pH= 0)
SiCl4(l) H2O(l) ——> SiO2.2H2O(s) HCl(aq) Si Cl
TETRAHEDRAL

14. 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) H2O(l) ——> H3PO4(aq) HCl(aq)

15. OXIDES OF PERIOD 3 ELEMENTS
SUMMARY
Na2O MgO Al2O SO2
Melting
point / K
bonding ionic ionic ionic/cov covalent
structure giant giant giant simple
lattice lattice lattice molecule
classification basic basic amphoteric acidic
solubility very sparingly insoluble reacts
in water
solution pH

16. 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)
2Mg(s) O2(g) ——> 2MgO(s)
4Al(s) O2(g) ——> 2Al2O3(s)
S(s) O2(g) ——> SO2(g)
+2 -2
+3 -2
+4 -2

17. 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) ——> NaCl(aq) H2O(l) Na + O 2- Na +

18. 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) ——> MgCl2(aq) H2O(l) Mg 2+ O 2-

19. 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
with acids Al2O3(s) + 6HCl(aq) ——> 2AlCl3(aq) H2O(l)
with alkalis Al2O3(s) + 2NaOH(aq) + 3H2O(l) ——> 2NaAl(OH)4(aq)

20. OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SO2 • simple covalent molecule
• soluble in water
• reacts with water to give a weak acidic solution (pH = 3)
SO2(g) H2O(l) H+(aq) + SO32-(aq)
• converted to sulphur trioxide in the CONTACT PROCESS O S
ANGULAR / BENT

21. OXIDES OF PERIOD 3 ELEMENTS
PROPERTIES
SO3 • simple covalent molecule
• reacts violently with water to give a strong acidic solution (pH = 0)
SO3(g) H2O(l) ——> 2H+(aq) + SO42-(aq) O S
TRIGONAL PLANAR

22. Mg(s) + H2O(l) ——> MgO(S) + H2(g)
METALS OF PERIOD 3
REACTION WITH WATER
Sodium • vigorous reaction
• hydrogen evolved
• strong alkaline solution produced (pH = 14)
Na(s) + 2H2O(l) ——> 2NaOH(aq) + H2(g)
Magnesium • 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)

23. © 2007 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
AN INTRODUCTION TO PERIOD 3 COMPOUNDS
THE END
© 2007 JONATHAN HOPTON & KNOCKHARDY PUBLISHING