Reading Exercise: The Halogens

Elements in group 7 of the periodic table are called the halogens. They share some similar properties (= features) and show a pattern in the way their properties change throughout the group.

All halogens exist as diatomic molecules where two atoms are held together by a chemical bond. They are non-metals which makes them poor conductors of heat and electricity. In addition, all halogens are poisonous and corrosive which means they burn through metals and living tissue like skin.

Physical properties

When going down group 7 melting points, boiling points and densities of the halogens increase. At room temperature, fluorine is a pale yellow gas and chlorine a green gas. Bromine is a brown liquid that evaporates easily. Iodine is a purple/black solid.

Chemical properties

Most halogens react with metals to form salts called metal halides, for example:

chlorine + magnesium -> magnesium chloride

fluorine + sodium -> sodium fluoride

The halogens become less likely to react in chemical reactions when going down the group. Their reactivity decreases. Fluorine is most reactive.

Displacement reactions

The order of the reactivity of halogens can be shown using displacement reactions where a more reactive element takes the place of a less reactive element in a salt.  For example, chlorine displaces bromine from sodium bromide in solution because chlorine is more reactive than bromine.

chlorine + sodium bromide à sodium chloride + bromine

However, bromine cannot displace chlorine from sodium chloride because it is less reactive.


All halogens can be used in disinfectants and bleaches because they kill microorganisms like bacteria and remove colour from materials. Chlorine is used as a disinfectant in swimming pools and in bleaches. Iodine is used to clean wounds.

Test for chlorine

If a damp, blue litmus paper is placed in chlorine gas, it first turns red then bleaches white.


  1. In which group of the periodic table are the halogens found?
  2. Halogens exist as diatomic molecules. What is meant by “diatomic”?
  3. Halogens are also corrosive. What is meant by “corrosive”?
  4. Describe what fluorine, chlorine, bromine and iodine look like at room temperature.
  5. What is formed when halogens react with metals?
  6. Chlorine reacts with iron. What is the name of the product formed in this reaction? Write the word equation.
  7. What is the trend in reactivity for halogens when going down the group?
  8. What happens in a displacement reaction?
  9. Why can chlorine displace bromine from sodium bromide?
  10. Give two uses for chlorine and one use for iodine.
  11. Describe the chemical test for chlorine.
  12. Fluorine is mixed with lithium iodide Can fluorine displace iodine from lithium iodide? Why? Write the word equation.


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