Group 7 – the halogens
The elements in group 7 of the periodic table, on the right,
are called the halogens.
F
fluorine
Cl
chlorine
Br
bromine
I
At
iodine
astatine
• Halogens form ions with a -1 charge
• Halides (e.g. Cl-) are negative ions which are made when halogen atoms
gain an electron. This happens when the halogens make compounds (e.g.
NaCl, KBr).
• Halogens all exist as DIATOMIC MOLECULES (i.e. F2, Cl2, Br2, I2) –
see next slide
• Halogens are all coloured elements
All the Group 7 elements are molecules containing two
atoms. (They are diatomic)
Each atom is 1 electron short of a noble gas electron
structure.
By sharing electrons in a covalent bond full outer
electron shells are achieved.
F
F
F
F
Halogens – what do they look like?
Chlorine
Bromine
Iodine
Element
Colour
Size
Melting
Point (oC)
Boiling
Point (oC)
Physical
State
Fluorine
Yellow
-220
-188
GAS
Chlorine
Green
-101
-35
GAS
Bromine
Orangebrown
-7
59
LIQUID
Iodine
Purple
+114
184
SOLID
Fluorine
Chlorine
Bromine
Iodine
As you go down the group:
• The halogens become darker in colour
• They become denser
• The melting and boiling point increases
• The radius of the atom increases
because there are more electron shells
• The elements become LESS reactive
Halogen vapours
Bromine and iodine are not gaseous, but have low boiling
points. This means that they produce vapour at
relatively low temperature. They are volatile.
Bromine produces some
orange-brown vapour,
seen here above, the
vapour diffuses up the
gas jar.
When iodine is heated gently, it
changes directly from a solid to
a gas without first becoming a
liquid.
This is called sublimation.
All the Group 7 elements have 7 electrons in
the outermost shell.
F
Cl
Chlorine
Br
Bromine
I
At
2,7
Fluorine
Iodine
2,8,7
And so on
Astatine
So, when group 7 elements react they need to GAIN an
electron to form a 1- ion.
BECOME LESS REACTIVE
WHY DO THE HALOGENS BECOME LESS REACTIVE
GOING DOWN THE GROUP?
As we go down the group:
The size of the atom increases because there are
more electron shells
So the outer shell electrons are further away from
the positive nucleus, so when an electron is added to
the halogen it is less attracted to the positive
nucleus
Also there is more shielding of inner shells so again
the electron is less attracted to the nucleus and so
the halogens become less reactive
Reactivity of fluorine
• The smaller the halogen atom the more strongly it
attracts an electron to form a halide ion.
Fluorine is more
reactive than the
other halogens
HALOGEN DISPLACEMENT REACTIONS
A less reactive halogen can be displaced by a less reactive
halogen from solution.
The organic solvent which dissolves the halogen is called
CYCLOHEXANE
Cyclohexane dissolves the halogen so that we can see the
colour of the halogen dissolved
SUMMARY OF DISPLACMENT REACTIONS
Halogen solution
Halogen
Potassium Potassium
chloride
bromide
Chlorine
x
Bromine
x
√
x
Iodine
x
x
X = reactions which do not work
√ = reaction which do work
Potassium
iodide
√
√
x
SUMMARY OF OBSEVATIONS MADE IN THE CYCLOHEXANE
Halogen solution
Halogen
Chlorine
Potassium Potassium
chloride
bromide
x
Orange/brown
colour seen
Bromine
x
x
Iodine
x
x
Potassium
iodide
Purple colour
seen
Purple colour
seen
x
Halogen displacement reactions
Halogen displacement reactions are redox reactions.
The equations below show the reactions that did work
CHLORINE + POTASSIUM BROMIDE  POTASSIUM CHLORIDE + BROMINE
Cl2 + 2KBr  2KCl + Br2
An orange brown
colour is seen in the
test tube because
bromine has been
displaced
The half equations for this reactions are:
Cl2 + 2e-  2Cl2Br-  Br2
(this is a reduction as electrons are gained)
(this is an oxidation as electrons are lost)
CHLORINE + POTASSIUM IODIDE  POTASSIUM CHLORIDE + IODINE
Cl2 + 2KI  2KCl + I2
The half equations for this reactions are:
Cl2 + 2e-  2Cl2I-  I2
(this is a reduction as electrons are gained
(this is an oxidation as electrons are lost)
A purple colour is
seen in the test tube
because iodine is
displaced
BROMINE + POTASSIUM IODIDE  POTASSIUM BROMIDE + IODINE
Br2 + 2KI  2KBr + I2
A purple colour is
seen in the test tube
since iodine has
been displaced
The half equations for this reactions are:
Br2 + 2e-  2Br2I-  I2
(this is a reduction as electrons are gained
(this is an oxidation as electrons are lost)
IONIC EQUATIONS FOR DISPLACEMENT REACTIONS
The ionic equations for the reactions that did work are shown
below:
Cl2 + 2Br-  2Cl- + Br2
Cl2 + 2I-  2Cl- + I2
Br2 + 2I-  Br- + I2
http://chemstuff.co.uk/2012/12/21/displacement-reactions/
The video above shows a displacement reaction carried out in a
laboratory
BONDING IN HALOGEN COMPOUNDS
Halogens can form both ionic and covalent compounds.
1.
REACTION OF HALOGENS WITH SODIUM
SODIUM + HALOGEN 
SODIUM HALIDE
EXAMPLES
Sodium + Chlorine  Sodium chloride
(2Na(s) + Cl2(g)  2NaCl)
Sodium + Bromine  Sodium bromide
(2Na(s) + Br2(l)  2NaBr(s))
Sodium + Iodine  Sodium iodide
(2Na(s) + I2(g)  2NaI)
This compound is ionic
and therefore has
strong electrostatic
forces of attraction
between opposite ions
i.e. strong ionic bonds
Ionic Bonding In NaCl
Na
Cl
-
+
Na
Cl
2. REACTIONS WITH HYDROGEN GAS
The halogens react with hydrogen gas to product hydrogen halides.
HYDROGEN + HALOGEN  HYDROGEN HALIDE
REACTIONS BECOME LESS
VIGOROUS GOING DOWN THE
GROUP
Fluorine explosive even at -2000C and in the dark
Chlorine and hydrogen explode in bright sunlight but react slowly in
the dark.
Bromine and hydrogen react slowly on heating with a platinum catalyst.
Iodine combines partially and very slowly with hydrogen, even on
heating with platinum catalyst.
Hydrogen halides are covalently bonded compounds. Covalent compounds
SHARE electrons:
H
Cl