Unit 1 Mod 3 Period 3 elements
Module 3 Period 3 elements
page 1 of 8
Variations in atomic/ionic radii, melting point and electrical conductivity of
period 3 elements
Atomic/ionic radii variation
Moving from left to right of the period, the atomic radii decrease. On
traversing the period there are 2 factors operating:- (1) the addition of
electrons to the same shell and (2) the number of protons increasing by one
thus increasing the effective nuclear charge.
What is effective nuclear charge? This is the residual attraction of the
nucleus to the outermost electrons after shielding from the inner core of
electrons have taken place.
By adding electrons, there is a slight repulsion between the core electrons
and the newly added electron causes a slight increase in the radius, but the
dominant effect is the effective nuclear charge, the positive charge attracts
the negatively charged electrons causing them to be drawn inward towards
the nucleus thus reducing the atomic radius.
For ionic radii, one must remember that in some cases cations are formed
and in other cases anions are formed. When cations are formed this
causes a reduction in the radius, while for anions there is repulsion
from added electrons therefore an increase in atomic radius is noted.
However in both anions and cations, the radius decrease from left to
right i.e. from Na to Al and from P to Cl.
Melting point variation
Showing the melting points of period 3 elements
Element
M.P /°C
Na
98
Mg
650
Al
660
Si
1410
P
44
S
114
Cl
-101
The first 3 elements are metals. Therefore metallic bonding would be the
factor involving the melting point temperature. One should appreciate that
as number of valence electrons increase, this results in a stronger metallic
bond. Moving from Na  Al, there is an increase from 1 to 3 valence
electrons, therefore stronger metallic bond i.e. increase in melting point.
Unit 1 Mod 3 Period 3 elements
page 2 of 8
Silicon is a non-metal, therefore type of bonding present is covalent bonding
between the silicon atoms. This results in a diamond-like structure which as
one knows from experience is a very strong structure. To break covalent
bonds, a lot of heat is needed i.e. a much higher melting point than any other
element.
From P  Cl, these represent non-metals, and have P4, S8 and Cl2 molecules
respectively. With discrete molecules, the factor responsible for melting
point temperature is the strength of the van der Waals’ forces. These weak
inter-molecular forces increase as the size of the atom/molecule increases,
and since it’s a cumulative effect i.e. the more van der Waals’ forces, the
more energy needed to break them. Therefore, the melting point drops
sharply at P, increases at S and then decreases from Cl through to Ar.
Electrical conductivity
All metal conduct electricity because of the presence of mobile electrons.
Electrical conductivity increases with increase number of valence electrons,
therefore Al is a better conductor than Mg, which in turn is a better
conductor than Na.
Silicon is a semi-conductor i.e. it can conduct current only in special
conditions. In all materials, conductors, semi-conductors and insulators
there are 2 types of bands:- valence band (where orbitals of metal atoms
overlap to form a cloud of electrons) and a conduction band (higher energy
level) Depending on conditions, electrons can jump from the valence to the
conduction band and conduct electricity.
In metals, there is no energy gap between the two bands, in insulators the
energy gap is too large and thus CANNOT conduct electricity under any
conditions. In semi-conductors, the energy gap is small enough to be
traversed under special conditions and be able to conduct electricity.
With P, S and Cl all are non-metals with no mobile electrons, therefore they
are all non-conductors.
Unit 1 Mod 3 Period 3 elements
page 3 of 8
Checkpoint A
1. Sketch a graph on the axes below to show the variation in ionic
radii for the period 3 elements
Reactions of period 3 elements with oxygen, water and chlorine
Reactions with water
Sodium
Sodium has a very exothermic reaction with cold water producing hydrogen
and a colourless solution of sodium hydroxide.
Magnesium
Magnesium has a very slight reaction with cold water, but burns in steam.
Magnesium burns in steam with its typical white flame to produce white
magnesium oxide and hydrogen.
Aluminium
Aluminium powder heated in steam produces hydrogen and aluminium
oxide. The reaction is relatively slow because of the existing strong
aluminium oxide layer on the metal, and the build-up of even more oxide
during the reaction.
Silicon
Most sources suggest that this form of silicon will react with steam at red
heat to produce silicon dioxide and hydrogen.
Phosphorus and sulphur:- These have no reaction with water.
Chlorine
A reversible reaction takes place to produce a mixture of hydrochloric acid
and chloric(I) acid (hypochlorous acid).
Argon
There is no reaction between argon and water.
Unit 1 Mod 3 Period 3 elements
page 4 of 8
Reactions with oxygen
Sodium
Sodium burns in oxygen with an orange flame to produce a white solid
mixture of sodium oxide and sodium peroxide.
For the simple oxide:
For the peroxide:
Magnesium
Magnesium burns in oxygen with an intense white flame to give white solid
magnesium oxide.
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 oxide is formed.
Silicon
Silicon will burn in oxygen if heated strongly enough. Silicon dioxide is
produced.
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.
The proportions of these depend on the amount of oxygen available. In an
excess of oxygen, the product will be almost entirely phosphorus(V) oxide.
For the phosphorus(III) oxide:
For the phosphorus(V) oxide:
Sulphur
Sulphur burns in air or oxygen on gentle heating with a pale blue flame. It
produces colourless sulphur dioxide gas.
Chlorine and argon
Despite having several oxides, chlorine won't react directly with oxygen.
Argon doesn't react either.
Unit 1 Mod 3 Period 3 elements
Reactions with chlorine
page 5 of 8
Sodium
Sodium burns in chlorine with a bright orange flame. White solid sodium
chloride is produced.
Magnesium
Magnesium burns with its usual intense white flame to give white
magnesium chloride.
Aluminium
Aluminium is often reacted with chlorine by passing dry chlorine over
aluminium foil heated in a long tube. The aluminium burns in the stream of
chlorine to produce very pale yellow aluminium chloride. This sublimes
(turns straight from solid to vapour and back again) and collects further
down the tube where it is cooler.
Silicon
If chlorine is passed over silicon powder heated in a tube, it reacts to
produce silicon tetrachloride. This is a colourless liquid which vaporises and
can be condensed further along the apparatus.
Phosphorus
White phosphorus burns in chlorine to produce a mixture of two chlorides,
phosphorus(III) chloride and phosphorus(V) chloride (phosphorus
trichloride and phosphorus pentachloride).
Phosphorus(III) chloride is a colourless fuming liquid.
Phosphorus(V) chloride is an off-white (going towards yellow) solid.
Sulphur
If a stream of chlorine is passed over some heated sulphur, it reacts to form
an orange, evil-smelling liquid, disulphur dichloride, S2Cl2.
Chlorine and argon
No reaction
Unit 1 Mod 3 Period 3 elements
page 6 of 8
Reactions of period 3 oxides/chlorides with water
Reaction with water
Oxide
Chloride
Na2O + H2O  2NaOH
NaCl + H2O  Na+(aq) + Cl-(aq)
pH 14
neutral solution
MgO + H2O  Mg(OH)2
MgCl2 + H2O  Mg2+(aq) + 2Cl-(aq)
pH 10
slightly acidic pH 6.5
Al2O3 Insoluble pH 7
AlCl3 + 6H2O  [Al(H2O)6]3+ + 3Clvery acidic pH 3
SiCl4 + 4H2O  Si(OH)4 + 4HCl
SiO2 Insoluble
very acidic pH 1
P4O10 + 6H2O  4H3PO4
PCl3 + 3H2O  H3PO3 + 3HCl
pH 1
very acidic pH 1
SO2 + H2O  H2SO3 pH 1
SO3 + H2O  H2SO4
PCl5 + H2O  POCl3
POCl3 + 3H2O  H3PO4
No reaction
Description and explanation of the acid/base behaviour of
oxides/hydroxides, including amphoteric behaviour in reaction with
NaOH and acids
The oxides of sodium and magnesium are basic, thus they form alkaline
solutions when dissolved in water as well as react only with acids.
Reaction: O2- + H2O  2OHNa2O + H2O  NaOH + H2O this occurs readily with sodium oxide
MgO is not as basic as the O2- is held tightly by the Mg2+ ion by still reacts
readily with acids. Note: MgO when placed in water forms a very weakly
basic solution.
Al2O3 does not dissolve in water as the O2- ion held too tightly by the Al3+
ion. However the oxide shows its amphoteric behaviour by reacting with
acids and bases.
Al2O3 + HCl  AlCl3 + H2O
Al2O3 + NaOH  NaAl(OH)4 + H2O
While the oxides of non-metals are acidic i.e. acidic solutions are formed
and would only react with bases.
In general, metallic (basic) character increases down a group and decrease across
(left to right) a period. Remember as the difference in electronegativities of the
elements and oxygen decreases, the oxide changes from a basic to amphoteric to an
acidic oxide.
Unit 1 Mod 3 Period 3 elements
page 7 of 8
NB:- °C = 273 + K ( K represents the Kelvin scale)
Oxide
Physical props
Na2O
Solid, sublimes
at 1548K
Chemical
props
Basic oxide
Chloride
NaCl
MgO
Solid, m.p at
3125K
Basic oxide
MgCl2
Al2O3
Solid with m.p
at 2323K
Amphoteric oxide
AlCl3
SiO2
Solid with m.p
at 1995K
Acidic
oxide
SiCl4
P4O10
Solid, sublimes
at 853K
Acidic
oxide
PCl3
SO2
Gas with m.p at
201K
Acidic
oxide
SCl2
Physical
props
Solid, m.p
at 1081K
Solid with
m.p at
988K
Solid with
m.p at
456K
Liquid
with m.p
at 203K
Liquid
with m.p
at 161K
Liquid
with m.p
at 195K
Chemical props
Good conductor
when molten or in
aq. soln
Good conductor
when molten or in
aq. soln
Poor conductor
when melted
Non-conducting
Non-conducting
Non-conducting
For the metallic elements excluding Al, the m.p. of the oxides/chlorides are high,
and their electrical conductivity when molten is good. This indicates the presence
of ions in their lattices therefore ionic bonding was present.
In the case of Al, the m.p. of the oxide is not as high as expected, as the high charge
density of the Al3+ ion pulls the electron cloud from the oxide ion towards itself
resulting in a high degree of covalent character. Therefore it has intermediate
ionic and covalent character, and shows amphoteric behaviour. In the chloride, the
high charge of the metal ion causes the electrons in the anion to be pulled towards
the cation forming a hybrid of ionic and covalent bonding, this results in partial
covalent bonding and ultimately a lower m.p than expected.
For the non-metallic chlorides/oxides, it is shown that the m.p of both compounds,
are low, they form acidic solutions and they have no electrical conductivity when
melted. All of this indicates that no ions are present and the bonds between the
particles are weak, the atoms within each molecule is bonded by strong covalent
bonds, while the inter-molecular forces are weak Van der Waals’ forces.
Unit 1 Mod 3 Period 3 elements
Checkpoint B
page 8 of 8
1. Give the pH values of the solutions when the period 3 chlorides listed are
placed in water [4]
NaCl
MgCl2
SiCl4
PCl5
2. Give the formulae of the oxides of the period 3 elements below (if there
is more than one oxide, you can write either one)
Na…………. [1]
Mg……….. [1] Si……….[1]
P………. [1]
3. Explain why Al would be more electrically conductive than Mg and why
the non-metals do not conduct electricity. [3]
………………………………………………………………………………
………………………………………………………………………………
………………………………………………………………………………
………………………………………………………………………………
………………………………………………………………………………
……………………………………………………………….
………………………………………………………………………………
………………………………………………………………………….
4. State the type of acid-base behaviour the oxides of the elements listed
show in the table below [5]
Oxide of
element
Acidic or basic
Na
MgO
Al2O3
SiO2
SO2
5. Write ONE equation each to show how Al2O3 react with
i) HCl ……………………………………………
ii) NaOH ………………………………………….
6. Complete the table below based on properties of period 3 chlorides.
Period 3
Melting point
Electrical conductivity of Type of
chloride
molten chloride
character
SiCl4
Poor
MgCl2
High
ionic
AlCl3
Poor