C1a.5.7 Group 7 – the Halogens

C1a.5.7 Group 7 – the Halogens
By the end of this lesson you should be able to:
Recall some of the properties of the halogens
Interpret data describing the properties of chlorine and iodine and explain
their uses
Properties of the Group 0 elements
The Group 7 elements have the following properties:
● Non-metals – low melting and boiling points, poor conductors of heat and
electricity
● Two are gases, one is a liquid and two are solids (at room temperature)
● Very reactive – hence the name ‘Noble’ but some do form compounds
under extreme conditions
● Exist as molecules (diatomic). The chemical formulae for halogen
molecules has a small 2 after the symbol, e.g. chlorine molecules, Cl2
Their reactivity decreases as you go down the group.
The halogens that are gases at room temperature are coloured. Those that are solids
or liquids form coloured vapours when heated:
Symbol
and
Name
State and colour at room temperature,
colour of vapour when heated
F
Fluorine
Cl
Chlorine
Br
Bromine
I Iodine
At
Astatine
Trend
in
colour*
Trend
in
state**
pale yellow gas
green gas
dark red liquid, brown vapour
dark crumbly solid, purple vapour
black solid, dark vapour
* The halogens become darker as you go down the group.
** The halogens change from gas to liquid to solid as you go down the group.
The halogens trend in state is due to their melting points and boiling points – these
both increase as you go down the group.
Uses of the halogens
All of the halogens are toxic and must be treated very carefully. However, this means
that they can be used for killing germs and bacteria, e.g. chlorine is used to kill germs
in drinking water during its treatment before being piped into homes. Chlorine is also
used to kill germs in water in swimming pools. Iodine is used in antiseptic washes and
to treat wounds.
Questions
1. Write the chemical symbols for fluorine, chlorine, bromine and iodine
2. Astatine is at the bottom of group 7. Suggest its colour and reasons for your
answer.
3. (a) What is the trend in the boiling points of the halogens?
(b) What would you expect the physical state of astatine to be at room temperature?
Explain your answer.
4. What does toxic mean?
5. What is a diatomic molecule?
6.The following table shows the melting and boiling points of each halogen:
Halogen
Melting
Boiling
Point (°C) Point (°C)
Fluorine
Chlorine
Bromine
Iodine
Astatine
327
445
539
660
848
358
512
605
730
883
Copy the table and add an extra
column ‘State at 120°C’
Complete the column for each
halogen.
7. Why would it be dangerous to put too much chlorine into the water in a swimming
pool?
Homework
1. The halogens are in group __________ of the periodic table. They are __________
conductors of __________ and electricity. The halogens are coloured and they exist as
pairs of __________ called __________ molecules. The boiling points of the halogens
__________ as you go down the group, and __________ is the only liquid non-metal.
__________ is used to sterilise __________ water and the water in __________ pools.
atoms bromine chlorine diatomic drinking heat increase poor swimming 7
2. At what temperature will all the halogens be solid?
3. What are the trends in colour, state and boiling point of the halogens as the atomic
number increases?
4. Table salt is sodium chloride, NaCl. It usually contains added potassium iodide or
potassium iodate. What are the chemical formulae for potassium iodide and
potassium iodate?