2 Chapter 6 • organising elements Organising elements

Halogens in the ocean
Element number 35 is bromine. It is one of the halogens
in group 17 of the periodic table. Bromine can control
bacterial growth and is used in the maintenance of
swimming pools. Bromine compounds are also used
as pesticides and as pharmaceuticals. Although the
concentration of bromide ions in sea water is very low,
bromine can be extracted from seawater on an industrial
scale. Chlorine gas is bubbled through water. Because
chlorine is more reactive than bromine, the chlorine
atoms are able to ‘take’ an electron from the bromide
ions (Br – ), becoming chloride ions (Cl – ), whereas the
bromide ions become neutral once they have lost that
‘extra’ electron. Bromine atoms, like chlorine atoms,
form molecules (Br 2 ). This process can be represented
by an equation, as shown below.
Cl 2 + 2 Br – → 2 Cl– + Br 2
Fig 6.42 Bromine compounds are used to
protect Australia’s borders from dangerous pests.
1 Use you knowledge of electrons and chemical
bonding to explain why group 17 elements, such as
bromine and chlorine, form ions with a charge of 1–.
2 Draw a labelled representation of a bromine molecule.
3 Look at the positions of chlorine and bromine in the
periodic table and suggest why chlorine is more
reactive than bromine.
4 Which member of the halogens (group 17) is the most
reactive? Explain your answer.
5 The pesticide methyl bromide has the formula CH 3 Br.
It is used by the Australian Quarantine and Inspection
Service (AQIS) to treat imported organic goods.
a Do you think that methyl bromide actually contains
bromide ions? Explain the reasons for your answer.
b Find out why the use of methyl bromide (or
bromomethane as it is also called) has been
restricted in a number of countries.
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