A2 CHEMISTRY NOTES - Periodicity
Trends in Period 3 Elements
Element
Na
Mg
Al
Si
P4
S8
Ar
Structure
Bonds
Melting Points
Boiling Points
Atomic Radii
Reactions of Sodium and Magnesium with water
Sodium
Sodium has a very exothermic reaction with cold water producing hydrogen gas and a
colourless solution of sodium hydroxide
2Na(s) + 2H2O(l)  NaOH(aq) + H2(g)
Magnesium
Magnesium has a very slight reaction with cold water, but burns in steam. A clean coil
of magnesium dropped into cold water eventually gets covered in small bubbles of
___________ gas which float it to the surface. Magnesium __________ is formed as
a very thin layer on the magnesium and this tends to stop the reaction.
Mg(s) + 2H2O(l)  Mg(OH)2 (aq) + H2(g)
Magnesium burns in steam with its typical _______ flame to produce white magnesium
oxide and hydrogen.
Mg(s) + H2O(g)  MgO (aq) + H2(g)
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A2 CHEMISTRY NOTES - Periodicity
Reactions of Sodium to Sulphur with Oxygen
Sodium
Sodium burns in oxygen with an _________ flame to produce a white solid mixture of
sodium oxide.
4Na(s) + O2(g)  2Na2O(s)
Magnesium
Magnesium burns in oxygen with an intense ______ flame to give white solid magnesium
oxide.
2Mg(s) + O2(g)  2MgO(s)
Aluminium
Aluminium will burn in oxygen if it is powdered, otherwise the strong oxide layer on the
aluminium tends to inhibit the reaction. If you sprinkle aluminium powder into a Bunsen
flame, you get white sparkles. White aluminium _______ is formed.
4Al(s) + 3O2(g)  2Al2O3 (s)
Silicon
Silicon will burn in oxygen if heated strongly enough. Silicon dioxide is produced.
Si(s) + O2(g)  SiO2 (s)
Phosphorus
White phosphorus catches fire spontaneously in air, burning with a white flame and
producing clouds of white smoke - a mixture of phosphorus(III) oxide and phosphorus(V)
oxide. In an excess of oxygen, the product will be almost entirely phosphorus(V) oxide.
P4(s) + 5O2(g)  P4O10 (s)
Sulphur
Sulphur burns in air or oxygen on gentle heating with a pale ______ flame. It produces
colourless sulphur dioxide gas.
S(s) + O2(g)  SO2 (g)
Chlorine and argon
Despite having several oxides, chlorine won't react directly with oxygen. Argon doesn't
react either.
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A2 CHEMISTRY NOTES - Periodicity
The oxides of Sodium to Sulphur
A quick summary of the trends
The oxides we'll be looking at are: Na2O MgO Al2O3 SiO2 P4O10 SO3 (Cl2O7)
These oxides are known as the _________ oxides of the various elements, where the
elements are in their highest ____________ states. In these oxides, all the outer
electrons in the Period 3 element are being involved in the _________ - from just the
____ with sodium, to all ________ of chlorine's outer electrons.
The structures
The trend in structure is from the metallic oxides containing giant structures of _____
on the left of the period via a giant _________ oxide (silicon dioxide) in the middle to
___________ oxides on the right.
Melting and boiling points
The giant structures (the metal oxides and silicon dioxide) will have _____ melting and
boiling points because a lot of ________ is needed to break the strong bonds (ionic or
covalent).
The oxides of phosphorus, sulphur and chlorine consist of individual _________ - some
small and simple. The attractive forces between these molecules will be __ ___ _____
and _______-_______ interactions. These vary in size depending on the ____, ____
and _________ of the various molecules - but will always be much weaker than the
ionic or covalent bonds you need to break in a giant structure. These oxides tend to be
_______, liquids or low melting point solids.
Electrical conductivity
None of these oxides has any free or mobile _________. That means that none of
them will conduct electricity when they are ______. The ionic oxides can, however,
undergo electrolysis when they are molten. They can conduct electricity because of the
movement of the ______ towards the electrodes and the discharge of the _____
when they get there.
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A2 CHEMISTRY NOTES - Periodicity
The metallic oxides
The structures
Sodium, magnesium and aluminium oxides consist of ______ structures containing metal
ions and oxide _____. Magnesium oxide has a structure just like sodium chloride. There
are strong attractions between the ____ in each of these oxides and these attractions
need a lot of heat energy to break. These oxides therefore have high melting and
boiling points.
Electrical conductivity
None of these conducts electricity in the solid state, but ________ is possible if they
are molten. They conduct electricity because of the movement of the ______ present.
Giant Covalent Oxide (Silicon dioxide)
The structure
There isn't enough electronegativity difference between the
silicon and the oxygen to form an ionic bond. Silicon dioxide is
a giant ________ structure. Crystalline silicon has the same
structure as _________.
Melting and boiling points
Silicon dioxide has a high melting point, around _______°C. Very strong silicon-oxygen
covalent bonds have to be broken throughout the structure before melting occurs.
Silicon dioxide boils at _______°C.
Electrical conductivity
Silicon dioxide doesn't have any mobile electrons or ions - so it doesn't conduct
___________ either as a solid or a liquid.
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A2 CHEMISTRY NOTES - Periodicity
The molecular oxides
Phosphorus, sulphur and chlorine all form oxides which consist of _________. Melting
and boiling points of these oxides will be much _____ than those of the metal oxides or
silicon dioxide. The intermolecular forces holding one molecule to its neighbours will be
___ ____ ________ forces or ________________ interactions. The strength of
these will vary depending on the ______ of the molecules.
None of these oxides conducts electricity either as solids or as liquids. None of them
contains _____ or free _____________.
Phosphorus(V) oxide
Phosphorus(V) oxide (P4O10) has a large formula ______ and so
the ___ ___ _______ bonds between molecules are relatively
_______ This makes it is a solid at room temperature
subliming at _____°C. .
Sulphur oxides
Sulphur has two common oxides, sulphur (__) oxide, SO2, and sulphur(__)
oxide), SO3 but again we will only be looking at the highest oxide.
Sulphur trioxide
Pure sulphur trioxide is a white solid with a ____ melting and boiling point. It
reacts very rapidly with water vapour in the air to form _________ acid.
Gaseous sulphur trioxide consists of simple SO3 molecules in which all ____ of the
sulphur's outer electrons are involved in the bonding.
There are various forms of solid sulphur trioxide. The
simplest one is a _______, S3O9, where _______ SO3
molecules are joined up and arranged in a ring. The ___ ____
_______ forces are weaker than those in P4O10 and so sulphur
trioxde is a ________ at room temperature
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A2 CHEMISTRY NOTES - Periodicity
Property
Formula
Oxide
Na2O
MgO
Al2O3
SiO2
P4O10
SO2
Particles
Bonding
Structure
Forces between
particles
Melting point
Boiling point
Electrical
conductivity
pH in water
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SO3
A2 CHEMISTRY NOTES - Periodicity
The trend in acid-base behaviour
The trend in acid-base behaviour is shown in various reactions, but as a simple summary:
The trend is from strongly _______ oxides on the left-hand side to strongly
________ ones on the right, via an ___________ oxide (aluminium oxide) in the
middle. An _______ oxide is one which shows both ______ and _____ properties.
Sodium oxide
Sodium oxide is a simple strongly ______ oxide. It is basic because it contains the oxide
ion, O2-, which is a very strong base with a high tendency to combine with _______ ions.
Reaction with water
Sodium oxide reacts exothermically with cold water to produce sodium ________
solution. Depending on its concentration, this will have a pH around ____.
Na2O (s) + H2O(l)  2NaOH (l)
Reaction with acids
As a strong base, sodium oxide also reacts with acids. For example, it would react with
dilute hydrochloric acid to produce sodium _________ solution.
Na2O (s) + 2HCl(aq)  2NaCl (aq) + H2O(l)
Magnesium oxide
Magnesium oxide is again a simple _____ oxide, because it also contains oxide ions.
However, it isn't as strongly basic as sodium oxide because the oxide ions aren't so
free. It takes more energy to break the attractions between 2+ and 2-. Even allowing
for other factors (like the energy released when the positive ions form attractions
with water in the solution formed), the net effect of this is that reactions involving
magnesium oxide will always be less ___________ than those of sodium oxide.
Reaction with water
Magnesium oxide doesn’t appear to react with water, however, if you test the pH of the
liquid, you find that it is somewhere around pH 9 - showing that it is slightly ________.
There must be some slight reaction with the water to produce hydroxide ions in
solution. Some magnesium hydroxide is formed in the reaction, but this is almost
insoluble - and so not many hydroxide ions actually get into solution.
MgO(s) + H2O(l)  Mg(OH)2(aq)
Reaction with acids
Magnesium oxide reacts with acids as you would expect any simple metal oxide to react.
It would react with warm dilute hydrochloric acid to give magnesium _______ solution.
MgO(s) + HCl(aq)  MgCl2(aq) + H2O(l)
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A2 CHEMISTRY NOTES - Periodicity
Aluminium oxide
Aluminium oxide is ____________. It has reactions as both a _____ and an _____.
Reaction with water
Aluminium oxide doesn't react in a simple way with water in the sense that sodium
oxide and magnesium oxide do, and doesn't dissolve in it. Although it still contains oxide
ions, they are held too ____________ in the solid lattice to react with the water.
Reaction with acids
Aluminium oxide contains oxide ions and so reacts with _______ in the same way as
sodium or magnesium oxides. That means, for example, that aluminium oxide will react
with hot dilute hydrochloric acid to give aluminium _________ solution.
l2O3(s) + 6HCl(aq)  2AlCl3(aq) + 3H2O(l)
In this (and similar reactions with other acids), aluminium oxide is showing the ______
side of its amphoteric nature.
Reaction with bases
Aluminium oxide has also got an acidic side to its nature, and it shows this by reacting with
bases such as sodium hydroxide solution. With hot, concentrated sodium hydroxide solution,
aluminium oxide reacts to give a colourless solution of sodium tetrahydroxoaluminate.
Al2O3 (s) + 2NaOH(aq) + 3H2O(l)  2NaAl(OH)4 (aq)
Silicon dioxide (silicon(IV) oxide)
By the time you get to silicon, electronegativity has increased so that there is no longer
enough electronegativity difference between silicon and oxygen to form ______ bonds.
Silicon dioxide has no basic properties - it doesn't contain oxide ions and it doesn't
react with acids. Instead, it is very weakly acidic, reacting with strong bases.
Reaction with water
Silicon dioxide doesn't react with water, because of the difficulty of breaking up the
giant covalent structure.
Reaction with bases
Silicon dioxide reacts with sodium hydroxide solution, but only if it is hot and
concentrated. A colourless solution of sodium silicate is formed.
SiO2(aq) + 2NaOH  Na2SiO3(aq)+ H2O(l)
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A2 CHEMISTRY NOTES - Periodicity
Phosphorus(V) oxide
Reaction with water
Phosphorus(V) oxide reacts violently with water to give a solution
containing a mixture of acids, the nature of which depends on the
conditions. We usually just consider one of these, phosphoric(_) acid,
H3PO4 - also known just as phosphoric acid.
Reaction with bases
Phosphoric(V) acid is also a weak acid with a pKa of 2.15. Acids of concentrations
around 1 mol dm-3 will have a pH of about 1. It has __ OH groups, and each of these
has an acidic hydrogen atom. If you were to react phosphorus(V) oxide directly with
sodium hydroxide solution rather than making the acid first, you would end up with the
same product.
Sulphur trioxide
Sulphur trioxide reacts violently with water to produce a fog of
concentrated ____________ acid droplets.
Pure un-ionised sulphuric acid has the structure shown. Sulphuric
acid is a strong acid, and solutions will typically have pH's of around __.
Sulphuric acid, of course, has all the reactions of a strong acid that you are familiar with.
For example, the normal reaction with sodium hydroxide solution is to form sodium
sulphate solution - in which both of the acidic hydrogens react with hydroxide ions.
Sulphur trioxide itself will also react directly with bases to form sulphates. For
example, it will react with calcium oxide to form calcium sulphate. This is just like the
reaction with sulphur dioxide described above.
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A2 CHEMISTRY NOTES - Periodicity
Oxide
Na2O
MgO
Al2O3
SiO2
P4O10
SO2
SO3
Typical pH
Structure
Bonding
Nature of Oxide
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