Halogens
Objectives
Be able to...
• Recall the main properties and trends of the
Halogens
• Explain trends using knowledge of
intermolecular bonding and redox
• Write half equations for displacement reactions
and electrolysis
Specification...
Where this comes from...
GROUP PROPERTIES
GENERAL
• non-metals
• exist as separate diatomic molecules… eg Cl2
• all have the electronic configuration ... ns2 np5
TRENDS
•
•
•
•
•
•
•
appearance
boiling point
electronic configuration
electronegativity
atomic size
ionic size
oxidising power
Physical properties of halogens
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Trends in boiling point
Halogen molecules increase in size down the group. This
leads to greater van der Waals forces between molecules,
increasing the energy needed to separate the molecules
and therefore higher melting and boiling points.
van der
Waals forces
fluorine
atomic radius = 42 × 10-12 m
boiling point = -118 °C
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iodine
atomic radius = 115 × 10-12 m
boiling point = 184 °C
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Trends in electronegativity
Electronegativity of the halogens decreases down the group
due to an increase in atomic radius.
Increased nuclear charge has no significant effect because
there are more electron shells and more shielding. Iodine
atoms therefore attract electron density in a covalent bond
less strongly than fluorine.
fluorine
atomic radius = 42 × 10-12 m
electronegativity = 4.0
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iodine
atomic radius = 115 × 10-12 m
electronegativity = 2.5
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Astatine
The name astatine comes from the Greek word for unstable.
Astatine exists in nature in only very tiny
amounts. It is estimated that only 30 grams of
astatine exist on Earth at any one time. This is
because it is radioactive, and its most stable
isotope (210At) has a half-life of only 8 hours.
It was first made artificially in 1940, by bombarding 209Bi with
a-radiation. What do you predict for these properties of
astatine?

colour

state at room temperature

electronegativity.
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Appearance
Halogen
Symbol
\
Fluorine
F
Chlorine
Cl
Bromine
Br
Iodine
I
State
Colour
Colour of
vapour
Appearance
Halogen
Symbol
State
Colour
Colour of
vapour
gas
Pale
yellow
Yellow
gas
Pale
green
Green
liquid
Orange /
brown
Orange
solid
Grey-black
crystals
Purple
\
Fluorine
Chlorine
Bromine
Iodine
F
Cl
Br
I
What would you predict about the appearance of astatine?
Physical Properties
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Iodine (I2)
What
happens to
the physical
properties
as you go
down
Group VII?
Trends in Physical Properties
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Iodine (I2)
• Melting points increase
• Boiling points increase
• All more soluble in organic
solvents than water
Make sure you can use
intermolecular forces to
EXPLAIN each of these trends
Reactivity of the Group 7 elements
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Iodine (I2)
• Decreasing reactivity
(This is due to it
getting less easy for
the atoms to form
negative ions by
gaining electrons)
Make sure you can
EXPLAIN this trend in
terms of atomic size,
shielding and nuclear
attraction
Displacement Reactions
Which of the following reactions will take place?
Chlorine
Bromine
Iodine +
Bromine
+ potassium bromide
+ potassium chloride
potassium chloride
+ potassium iodide
What has to be true for a displacement
reaction to take place?
Electron structure and reactivity
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Halogen displacement reactions
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Halogen displacement reactions
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Halogen displacement reactions
Halogen displacement reactions are redox reactions.
Cl2 + 2KBr  2KCl + Br2
To look at the transfer of electrons in this reaction, the
following two half equations can be written:
Cl2 + 2e-  2Cl-
2Br-  Br2 + 2e-
What has been oxidized and what has been reduced?

Chlorine has gained electrons, so it is reduced to Cl- ions.

Bromide ions have lost electrons, so they have been
oxidized to bromine.
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In displacement reactions between
halogens and halides, the halogen
acts as an oxidizing agent.
This means that the halogen:

oxidizes the halide ion to the
halogen

gains electrons

is reduced to form the halide ion.
What is the order of oxidizing
ability of the halogens?
increasing oxidizing ability
Oxidizing ability of halogens
fluorine
chlorine
bromine
iodine
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Oxidizing ability of halogens
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Displacement Reactions
Writing an ionic equation for a displacement reaction:
Br2 + 2I
2Br + I2
• Which species is oxidised in this reaction?
• Which species is the oxidising agent?
• Which species is the reductant?
Displacement Reactions
Chlorine can oxidise both bromide and iodide ions:
Cl2 + 2BrCl2 + 2I
2Cl- + Br2
2Cl + I2
However, bromine can only oxidise iodide ions:
Br2 + 2Cl
Br2 + 2I-
no reaction
2Br- + I2
So, chlorine is a stronger oxidising agent than bromine.
Oxidising ability of the halogens
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Iodine (I2)
Decreasing
oxidising ability
Decreasing
reactivity
Reducing ability of the halides
(F )
Fluoride
Chloride (Cl )
Bromide (Br )
Iodide (I )
Increasing
reducing ability
GROUP TRENDS
ELECTRONEGATIVITY
Electronegativity
F
Cl
Br
I
4.0
3.5
2.8
2.5
DECREASES down Group
• the increasing nuclear charge due to the greater number of protons
should attract electrons more, but there is an ...


an increasing number of shells;
more shielding and less pull on electrons
an increasing atomic radius
attraction drops off as distance increases
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
• the trend can be explained by considering the nucleus’s attraction
for the incoming electron which is affected by the...
• increasing nuclear charge which should attract electrons more
but this is offset by
• INCREASED SHIELDING
• INCREASING ATOMIC RADIUS
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
• the trend can be explained by considering the nucleus’s attraction
for the incoming electron which is affected by the...
• increasing nuclear charge which should attract electrons more
but this is offset by
• INCREASED SHIELDING
• INCREASING ATOMIC RADIUS
This is demonstrated by reacting the halogens with other halide ions.
Reactions with Silver Nitrate
Halide
Colour of
precipitate
Formula of
precipitate
Solubility in
ammonia
Fluoride
no ppt.
no ppt.
soluble
Chloride
Bromide
Iodide
white
cream
yellow
AgCl
AgBr
AgI
soluble
partially soluble
insoluble
How is AgNO3 used to test for halide ions?
Which silver halide precipitate is formed most quickly?
Can you suggest a use for the silver halides?