- Halogens are Group 7 non-metals, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
- Halogens are diatomic, which means they form molecules of two atoms. In nature, halogens always exist as F2, Cl2, Br2, I2 and At2.
Properties and Trends of Halogens
- Colour and state of halogens at room temperature: As halogens go down the group, melting point and boiling point increases. There is a trend in state from gas to liquid to solid as you go down the group. The colour also becomes darker as you go down the group.
For IGCSE Chemistry, you should be able to predict the colour and state of halogens based on the trend.
- Reactivity of halogens: Reactivity of halogens increases up the group. Fluorine is the most reactive while astatine is the least reactive.
- Halogens have 7 electrons in their outer electron shells.
- When halogens react, they need to gain one electron to have a full outer shell.
- As halogens move up the group, the number of electron shells decreases.
- It is easier to attract electrons to the outer shell with fewer number of electron shells.
- Therefore, reactivity of halogens increases up the group.
Displacement Reactions Involving Halogens and Halides
A more reactive halogen displaces a less reactive halogen from a solution of its halide.
- When chlorine is added to a potassium iodide solution, the more reactive chlorine will replace the less reactive iodine. (Cl2 + 2KI → I2 + 2KCl)
- When bromine is added to a sodium fluoride solution, no reaction will occur since bromine is less reactive than fluorine.
Each of the halides were dissolved in water to form a solution, and a sample of each of the halogens was added to the halide solution.
(a) Why was there no reason to add chlorine to lithium chloride?
No displacement reaction will take place.
(b) Why was there no reaction when iodine was added to sodium bromide solution?
Iodine is less reactive than bromine, so there is no displacement reaction.
(c) Name the substance with the brown colour that formed when chlorine was added to potassium iodide solution.
(d) Which halogen displaces which halogen in the reaction between bromine and potassium iodide solution?
Bromine displaces iodine.
(e) Write the chemical equation for the reaction between chlorine and potassium bromide solution.
Cl2 + 2KBr → Br2 + 2KCl