OCR Advanced GCE in Chemistry/Periodic Table: Period 3

In this section there is a large amount of information and unfortunately most of it just has to be learnt. Read the suggestions at the end of each sub-section for best ways of learning.

Period 3 Oxides


You are required to know about the formation and reactions of MgO, Al2O3 and SO2

Magnesium oxide

Ionic Lattice

MgO is formed simply by heating in oxygen:

Mg(s) + 1/2 O2(g) ---> MgO(s)

Observations: Burns with bright white flame to form white solid. (used in fireworks for this reason)

Basic oxide: Magnesium oxide is a basic oxide, meaning that when it dissolves in water an alkaline solution will be made:

MgO(s) + H2O (l) ---> Mg(OH)2 (aq)

Mg(OH)2 is an weak alkali due to partial dissociation of ions.

Structure: Magnesium oxide forms a giant lattice structure with ionic bonding

Aluminium Oxide

Aluminium Oxide

Aluminium reacts very easily in oxygen, so much so that at room temperature it forms a layer around aluminium. Once this layer has formed, it protects the aluminium and prevents it from readily reacting. However, if the aluminium is in powdered form the aluminium burns. The formation reaction is as follows:

4Al(s) + 3O2(g)---> 2Al2O3(s)

Observations: Burns when powdered - white solid product

Amphoteric oxide: This means that it can react as either an acid or a base (see below). However, aluminium oxide will not react with water (insoluble).

Structure: Aluminium Oxide has 'intermediate bonding'- This basically means that it has a Giant lattice structure but also has some covalent bonding too. The intermediate bonding in aluminium oxide explains why it is insoluble.

Sulphur dioxide molecule

Sulphur dioxide


Sulphur burns in oxygen to produce sulphur dioxide:

S(s) + O2(g) ---> SO2(g)

Observations: Burns with blue flame to give gas

When heated with a platinum catalyst SO3(l) is formed

Acidic oxide: Sulphur dioxide reacts with water leaving an acidic solution:

SO2(l) + H2O(l) ---> H2SO3(aq)

and similarly:

SO3(l) + H2O(l) ---> H2SO4(aq)

Structure: Simple molecular

Acid-base reactions


Some questions ask you to give examples of "acid-base reactions" they may or may not say oxides specifically. Basically an acid-base reaction is one where an acid and a base react with each other. We have already discussed the nature of these oxides (e.g. acidic oxide - sulphur) and this refers to the role that it plays in the reaction:

MgO(s) + 2HCl(aq) ---> MgCl2(aq) + H2O(l)

Here, magnesium oxide acts as a base in a reaction with hydrochloric acid.

Aluminium oxide is amphoteric so acts as a base or an acid. Its reaction as a base is similar to above but its reaction as an acid occurs when it is hydrated:

Al(OH)3(s) + OH-(aq) ---> [Al(OH)4]-(aq)

It is also worth knowing the acid-base reaction for sulphur because it is a little different to what may be expected. We already know that sulphur is an acidic oxide and that it is able to form SO3, and now we can see this in action:

SO3(g) + NaOH(aq) ---> Na2SO4(aq) + H2O(l)

The OH group is removed in the reaction as SO3 is acting as an acid.


  • Basic oxides (left of periodic table) have giant lattice structures and form alkaline solutions in water
  • Amphoteric oxides (aluminium oxide - metalloid) have giant lattice structure with some covalent bonding so don't dissolve
  • Acidic oxides (right of periodic table) are simple molecules and react with water to produce acid

Period 3 Chlorides


The chlorides that you need to know about are NaCl, MgCl2, AlCl3, SiCl4 and PCl5

Solid Magnesium Chloride

Sodium and Magnesium Chlorides


2Na(s) + Cl2(g) ---> 2NaCl(s)

Mg(g) + Cl2(s) ---> MgCl2(s)

There is nothing fancy about these reactions or products - you simply heat the metals and lower into chlorine gas to form white solids.

Structure and bonding - Both of these products have giant ionic lattice structures with ionic bonding

Reactions with water (products) - Sodium chloride dissolves in water, leaving a neutral solution. Magnesium chloride also dissolves but leaves a slightly acidic solution.

Aluminium trichloride

Aluminium Chloride


Some text books miss out Aluminium chloride but the specification includes it with the other chlorides so it is important. Aluminium Chloride can be prepared under anhydrous conditions by heating aluminium in dry chlorine gas:

2Al(s) + 3Cl2(g) ---> 2AlCl3(s)

When AlCl3 is heated above its melting point of 178 °C (451 K) it forms a dimer with formula Al2Cl6 where the chlorine atoms donate electron pairs forming dative covalent bonds. At higher temperatures the Al2Cl6 dimer dissociates into trigonal planar AlCl3

Structure - Aluminium trichloride has simple molecular structure with covalent bonding

Reaction with water - Hydrolysis reaction evolving hydrogen chloride gas, some dissolves to leave acidic solution:

Al2Cl6(s) + 6H2O(l) ---> 2Al(OH)3(s) + 6HCl(g)

Silicon tetracloride

Silicon tetrachloride


Silicon tetrachloride is prepared in a similar way to aluminium chloride except a side arm is needed because silicon tetrachloride is a liquid at room temperature.

Si(s) + 2Cl2(g) ---> SiCl4(l)

Structure - Molecular

Reaction with water - Hydrolysis:

SiCl4(l) + 2H2O(l) ---> SiO2(s) + 4HCl(g)

Observations - white fumes of hydrogen chloride gas

Phosphorus pentachloride

Phosphorus pentachloride


Phosphorus trichloride combines directly with chlorine without heating:

P4(s) + 6Cl2(g) ---> 4PCl3(l)

However, we are not really interested in the trichloride but the solid pentachloride that is formed in excess chlorine:

PCl3(l) + Cl2(g) ---> PCl5(s)

Stucture - Covalent molecular in gaseous state

Reaction with water - Hydrolysis, leaving acidic solution:

PCl5(s) + H2O(l) ---> 5HCl(g) + H3PO4(aq)


  • Neutral chlorides (left of periodic table) have giant lattice structures and dissolve to give neutral solutions (or very slightly acidic)
  • Acidic chlorides (right of periodic table) have simple molecular structure and undergo hydrolysis reactions to leave acidic solutions

Reactions of period 3 elements with water




Sodium reacts vigorously with water, fizzing around the surface and sometimes catching fire:

2Na(s) + 2H2O(l) ---> 2NaOH(aq) + H2(g)

The product sodium hydroxide is a strong alkali with a high PH (>12) sodium is a very reactive element in the periodic table



The reaction of magnesium with water is much slower than sodium, hydrogen bubbles form very slowly:

Mg(s) + 2H2O(l) ---> Mg(OH)2 + H2(g)

This alkali is much weaker than sodium hydroxide (PH 9-11) This is because sodium hydroxide dissolves more easily - more ions in the solution.

However, if magnesium is reacted with steam the reaction is much faster in forming the oxide:

Mg(s) + H2O(g) ---> MgO(s) + H2(g)



(These questions are easy if you look at the page so do it without to check you know everything)

1. Give an example of hydrolysis and acid-base reactions. Show equations and state observations

2. State the type of bonding in Aluminium oxide, does it dissolve?

3. Which reacts most readily with water - sodium or magnesium?

4. How can magnesium be made to react with water?

5. How does one make PCl5?

6. Write out all 21 equations shown on this page...(no seriously)