The halogens, fluorine, chlorine, bromine, iodine are reactive nonmetallic elements of the p block. They are p block elements because
their valence electrons occupy a p-sub shell. We only consider the
elements chlorine, bromine and iodine in our treatment of the
halogens: The Physical Properties of the elements
Halogen
Fluorine
Chlorine
Bromine
Iodine
Astatine
Physical State
(Gas)
(Gas)
(Liquid)
(Solid)
(Radioactive solid)
Electronic Configuration
1s2 2s2 2p5
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
[Kr] 4d10 5s2 5p5 (not examined)
[Xe] 4f14 5d10 6s2 6p5 (not examined)
 Bond type in the halogen
molecule______covalent________________
 Exist as ________diatomic____ molecules
Draw a cross-dot diagram of chlorine, Cl2 in the space below
 Structure is ___simple____________ molecular
 The intensity of the colour of the halogens
____increases__________ down the group.
 Volatility _____decreases_______ down the group.
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What do you understand by the term volatility?
This term describes how easily a substance turns into a gas
The group seven elements are described as being volatile and the
elements become less volatile down the group.
This means that the boiling points of the halogens increase down the
group. (Obvious really as chlorine is a gas, bromine is a liquid and
iodine is a solid).
As we descend group seven the intermolecular forces between the
molecules of the halogens increases. These intermolecular forces are
known as
_________Van der Waal’s_________________.
The strength of these forces results from interactions of electrons
between the molecules. The more electrons in the molecule the
greater the induced diploes and the greater the attraction between
the molecules. These increased attractions cause the boiling points to
increase as these forces need to be broken.
Chlorine
Bromine
Iodine
Boiling point increases as the van der Waals
forces increases due to the molecules containing
more electrons. The van der waals forces
become more difficult to break
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Reactivity of the halogens
The halogens all have seven electrons in their outer shells. A
halogen atom readily accepts an electron to form a halide ion with a
full shell of electrons (very stable)
Reacting as oxidising agents
All of the halogens act as oxidising agents. The most powerful is
____chlorine________.
State and explain the trend in oxidising power down the group
………Decreases
Less readily gains an electron due to decreased effective attraction of
the nucleus for the electrons. Distance from nucleus of outer
electrons is greater because number of shells and shielding is
increasing down the group
……………………………………………………………………………..
……………………………………………………………………………..
……………………………………………………………………………..
State and explain the trend in reactivity down the group
Decreases
……………………………………………………………………………..
Atomic radius increases
Electron shielding increases
The ability to gain an e into the p sub shell dereases ( to form a halide
ion
Effective nuclear attraction is weaker
……………………………………………………………………………..
……………………………………………………………………………..
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You must be able to state and explain trends
down group VII – look at the Group II
booklet for similarities and differences.
Don’t forget about atomic radii – this applies
to group II and group VII.
The decrease in reactivity of the halogens as the group is descended
can be demonstrated by displacement reactions of the aqueous
halides using chlorine, bromine and iodine.
When chlorine is added (bubbled through) a solution of potassium
iodide, the solution becomes brown due to the formation of iodine. If
bromine is added to another solution of potassium iodide the same
reaction would be seen. In both of these reactions the iodide ion is
being oxidized to iodine
I2(aq) + 2e-
2I-(aq)
A fully balanced equation can be written for both of these reactions
Br2 (g)
+ 2I-(aq)
2Br-(aq)
+ I2(aq) **
Cl2 (g)
+ 2I-(aq)
2Cl-(aq)
+ I2(aq) **
In both the above reactions iodide has been oxidized. This occurs
because bromine and chlorine are stronger oxidizing agents than
iodine. Iodine being the weakest oxidizing agent could not displace
any of the others from their solutions.
When chlorine is added to a solution of potassium bromide, the
solution becomes yellow orange as bromine is released. This reaction
shows that chlorine is a better oxidizing agent than bromine.
This can be seen in the equation below
Cl2 (g)
+ 2Br-(aq)
2Cl-(aq)
4
+ Br2(aq)
Be careful with terminology – nothing reacts with
chloride. Watch for the traps in the questions!
**Iodine in a non polar solvent is purple – watch out!
5
The halide ion can be determined in aqueous
solution by the use of a solution of silver nitrate
followed by the addition of dilute ammonia solution.
When a solution of silver nitrate is added to a halide ion in solution
the following observations are recorded. After the addition of the
silver nitrate, ammonia solution is added to any of the precipitates
formed.
HALIDE ION
OBSERVATIONS
WITH
SILVER NITRATE
OBSERVATIONS
WITH
AMMONIA
Soluble ( dilute ammonia)
Cl-
White ppt
-
Cream ppt
Soluble( conc ammonia)
Br
Yellow ppt
I
Insoluble
-
Equations
Write equations for the reactions between the
halide ions and silver ions
Cl- (aq)+ Ag+(aq) → AgCl(s)
Br- (aq)+ Ag+(aq) → AgBr(s)
I- (aq)+ Ag+(aq) → AgI(s)
………………………………………………………………..
……………………………………………………………………………..
……………………………………………………………………………..
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Reactions of the halogens with sodium hydroxide
Chlorine, bromine and iodine disproportionate in alkali.
Write an equation here for the reaction between NaOH and chlorine:
2NaOH + Cl2 →NaCl + NaOCl + H2O
Also in hot conc alkali
This is on Jan 10 paper Q4d
For example in cold alkali (15OC) chlorine disproportionates to give
chloride and chlorate ions. Chlorate is the ion
present in bleach.
In the equation the Oxidation State of the chlorine is
given below the species in the balanced equation.
Cl2 (g) + 2OH- (g)
(0)
Cl - (aq) + ClO-(aq) + H2O(l)
(-1)
(+1)
Chlorine in water is both an acid and a bleach - watch your teacher
demonstrate the action of chlorine water on indicator paper – there
are two changes________________________________________
Chlorine reacts with water. This reaction is used in water
purification. A disproportionation reaction occurs and hydrochloric
acid and chloric(I) acid is formed
Cl2 (g)
+
H2O (l)
HCl(l)
+
HOCl (l)
The bacteria in water are destroyed by reactive oxygen atoms which
are produced by a slow decomposition reaction of the chloric acid =
sterilisation
HClO
HCl
+ O
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Some people think we should not put chlorine in drinking water – why? Chlorine
is toxic in excess and it can react with hydrocarbons to form chlorinated
hydrocarbons which are carcinogens.
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