Group VII
Chlorine to iodine
Physical properties
 They are all non-polar molecules.
 Except fluorine, they are all slightly soluble in
water. Their solubility decreases as we go down
in the group.
 Halogens are more soluble in organic solvents.
Their solubility increases as we descend in the
group.*
 They are all coloured, their colours become
darker as we go down in the group.
 Their melting/boiling points increase as we
descend in the group.
Chemical properties
They are strong oxidising agents, as
shown in their reactions with metals.
Although their most common oxidation
state is -1, they have various oxidation
numbers in their compounds (except F).
They occur naturally as compounds with
metals (as metal halides), this is why they
are called halogens (salt makers in ancient
Greek).
Reactions of the halogens
 Halogens will remove e- from any species that is
less electronegative than them*
For instance, halogens remove e- from metals
and some metal ions (e.g. Fe2+). In other
words, they increase their oxidation number.
Halogens themselves gain e-, their oxidation
number is decreased.
e.g. 2K + Cl2 → 2KCl or 2Fe2+ + Br2 → 2FeBr3 *
 Halogens will also remove e- from halide ions
as long as the halide comes from a halogen that
is less reactive.
e.g. 2KBr + Cl2 → 2KCl + Br2*
 Halogens will react with other non-metals, but
they will form a polar covalent bond as their
difference in electronegativity is equal to or
smaller than 1.
e.g. 2P + 3Cl2 → 2PCl3 or H2 + Br2 → 2HBr
neither of these bonds is ionic as we saw before
The speed of all of these reactions depends on
the reactivity of the halogen. This is particularly
well demonstrated in the reaction of halogens
with hydrogen*.
Disproportionation reactions of the halogens
 When one of the halogens is reacted with OHthe following reaction occurs:
X2 + 2OH- → X- + XO- + H2O
When performing this reaction in the lab we will
see the colour of the halogen solution disappear.
 Rate of reaction will depend on the reactivity of
the halogen and temperature.*
 On heating, XO- disproportionates to XO3- and
X- :
3XO- → XO3- + 2XSo when an halogen reacts with hot OH-:
3X2 + 6OH- → XO3- + 5X- +3H2O
Reactions of halide ions
Reactions of halides with conc. H2SO4
 Conc H2SO4 reacts with a chlorides to form hydrogen
chloride.
MgCl2 + 2H2SO4 → Mg(HSO4)2 + 2HCl
 In the case of bromides and iodides, the acid acts as an
oxidising agent
2HBr + H2SO4 →Br2 + 2H2O + SO2*
 The iodides are oxidised more easily (and the acid
reduced further) so in their reaction with H2SO4 the acid
is reduced to H2S.
 H2SO4 + 8H+ + 8I- → H2S + 4H2O + 4I2*
Testing for halide ions
 Silver nitrate is added to the solution to form silver
halides. Then acid is used to prevent the formation of
other silver salts as they dissolve in the presence of H+
ions.*
 The precipitate is then tested by adding NH3.
The technique is based on the different
solubilities in NH3 that these halides display*
Hydrogen halides
Reactions hydrogen halides with ammonia
 They react to form a salt*
Reactions of hydrogen halides with water
 They react to form the corresponding acidic
solution.*