1. 13.1 REACTIONS OF PERIOD 3 ELEMENTS
GROUP 3

2. 13.1 REACTIONS OF PERIOD 3 ELEMENTS
Atomic Radius
– Decreases across period
Extra protons pull electrons closer
No extra shielding
Melting/Boiling Point
Increase up to silicon then decrease
Metallic bonding in Na, Mg and Al
Macromolecular tetrahedral structure in Si
P, S, Cl are all molecular – M.P & B.P depend on VdW
Ar exists as individual atoms
First Ionisation Energy -

3. 13.1 REACTIONS OF PERIOD 3 ELEMENTS
REACTIONS WITH WATER
Sodium:
Reacts vigorously forming a molten ball on the surface, fizzing and producing H2 gas
2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
Magnesium:
Reacts slowly with no visible reaction
It forms a weakly alkaline solution  how we know it reacts
Mg(s) + 2H2O(l)  Mg(OH)2(aq) + H2(g)

4. 13.1 REACTIONS OF PERIOD 3 ELEMENTS
WHY DOES SODIUM REACT MORE?
When they react:
Na loses 1 electron
Mg loses 2 electrons
It takes less energy to remove 1 electron than it does 2
More energy (usually heat) is needed for Mg to react
Mg(OH)2 is not very soluble in water, so produces relatively few OH- ions. This makes it a weaker alkali than NaOH

5. 13.1 REACTIONS OF PERIOD 3 ELEMENTS
REACTIONS WITH OXYGEN
Element
Oxide
Oxidation State of Element
Extent of Reaction
Flame Na
Na2O +1
Vigorous
Yellow Mg
MgO +2
Brilliant White Al
Al2O3 +3
Slow
N/A Si
SiO2 +4 P
P4O10 +5
One allotrope* spontaneously combusts; the other requires heating S
SO2
Burns steadily
Blue
*Allotrope: same element but atoms arranged differently

6. 13.2 OXIDES OF ELEMENTS IN PERIOD 3
TRENDS IN THE PHYSICAL PROPERTIES
Metal Oxides  Na2O, MgO & Al2O3
High melting points  giant ionic lattices
Strong forces of attraction between each ion
Lots of heat energy required to break bonds
MgO is higher than Na2O because 2+ ions attract O2- ions more strongly than 1+ sodium ions
Al2O3 is lower than expected because 3+ ions are small with a high charge  this distorts oxygen’s electron cloud making the bond partially covalent

7. 13.2 OXIDES OF ELEMENTS IN PERIOD 3
TRENDS IN THE PHYSICAL PROPERTIES
Non-metal Oxides  SiO2, P4O10 & SO2
SiO2 is highest of non-metal oxides:  forms a giant macromolecular structure  Strong bonds that require lots of energy to be broken
P4O10 & SO2 are simple covalent molecules  Weak intermolecular forces  Dipole-Dipole forces  Van der Waals forces

8. 13.2 OXIDES OF ELEMENTS IN PERIOD 3
Ionic oxides of Na & Mg dissolve to form hydroxides
Form alkaline solutions
Sodium hydroxide is more soluble in water, so forms a more alkaline solution than magnesium hydroxide
Na2O(s) + H2O(l)  2NaOH(aq) pH 12-14
MgO(s) + H2O(l)  Mg(OH)2(aq) pH 9-10

9. 13.2 OXIDES OF ELEMENTS IN PERIOD 3
Simple covalent oxides (P and S) form acidic solutions
Phosphorous (V) oxide reacts violently to form phosphoric acid:
P4O10(s) + 6H2O(l)  4H3PO4 (aq)  This ionises:
4H3PO4 (aq) H+(aq) + H2PO4-(aq)
Sulphur dioxide reacts to form sulphuric (IV) acid:
SO2(g) + H2O(l)  H2SO3(aq)  partially dissociates
H2SO3 (aq) H+(aq) + HSO3-(aq)

10. 13.2 OXIDES OF ELEMENTS IN PERIOD 3
Giant covalent structure of silicon dioxide means it is insoluble. It will react with bases to form salts so is classed as acidic
Aluminium oxide is also insoluble. It will react with acids AND bases to form salts. i.e. it can act as an acid OR a base  AMPHOTERIC
Sulphur trioxide reacts violently to form sulphuric (VI) acid:
SO3(g) + H2O(l)  H2SO4(aq)  H+(aq) + HSO4-(aq)

11. 13.3 OXIDES WITH ACIDS & BASES
pH OF PERIOD 3 OXIDES:
P4O10
MgO
SO2
Al2O3
Na2O
SiO2
SO3

12. 13.3 OXIDES WITH ACIDS & BASES
A CLASSIC
The equation for neutralising an acid with a base is a classic
Acid + Base  Salt + Water
It’s no different for Period 3 oxides
You will be expected to write the equations

13. 13.3 OXIDES WITH ACIDS & BASES
SODIUM & MAGNESIUM:
These oxides are basic so will neutralise acids.
E.g. Sodium oxide reacts with hydrochloric acid to form Sodium chloride and water
Na2O(s) + 2HCl(aq)  2NaCl(aq) + H2O(l)
E.g. Magnesium oxide reacts with sulphuric acid to form magnesium sulphate and water
MgO(s) + H2SO4(aq)  MgSO4(aq) + H2O(l)

14. 13.3 OXIDES WITH ACIDS & BASES
ALUMINIUM OXIDE:
This is an amphoteric oxide  it can react with both acids and alkalis
E.g. With sulphuric acid, aluminium sulphate is formed
Al2O3(s) + 3H2SO4(aq)  Al2(SO4)3 (aq) + 3H2O(l)
E.g. With hot, concentrated sodium hydroxide, sodium aluminate is formed
Al2O3(s) + 2NaOH(aq) + 3H2O(l)  2NaAl(OH)4(aq)

15. 13.3 OXIDES WITH ACIDS & BASES
SILICON, PHOSPHOROUS & SULPHUR:
These oxides are all acidic so will neutralise bases
SiO2(s) + 2NaOH(aq)  Na2SiO3(aq) + H2O(l)
P4O10(s) + 12NaOH(aq)  4Na3PO4(aq) + 6H2O(l)
SO2(g) + 2NaOH(aq)  Na2SO3(aq) + H2O(l)
SO3(g) + 2NaOH(aq)  Na2SO4(aq) + H2O(l)
Phosphorous (V) Oxide (Phosphorous pentoxide) is actually more complicated than this. Read the bottom of page 192 and write out the steps. Start by reacting P4O10 with H2O
Lewis Acids  electron pair acceptors Lewis Bases  An electron pair donor