3.2.3 Group 7: The Halogens

• The group 7 elements are highly reactive non-metals which need to gain an electron to form a 1- ion and achieve a full outer shell of electrons.
• The atomic radius of group 7 increases down the group due to additional shells and electrons.
• Reactivity decreases down group 7 because atomic radius increases and shielding increases which weakens attraction.
• The first ionisation energy decreases down group 7 because atomic radius increases as does shielding.
• The boiling point of group 7 elements increases down the group because they have a simple molecular crystal structure held together by van Der Waals forces which increase down the group meaning more energy is required to overcome them.
• The halogens act as good oxidising agents as they accept electrons from the species being oxidised and are reduced.
• The oxidising power of group 7 halogens decreases down the group as their ability to attract electrons decreases due to shielding and greater atomic radius.
• The relative oxidant strengths of group 7 means that a halogen will displace any halide beneath it in the periodic table.
• The negative ions of halogens are known as halides.
• Halides are good reducing agents as they donate electrons to the species being reduced and are oxidised themselves.
• The reducing power of halides increases down the group because electrons are easier to lose from larger ions due to shielding and a greater atomic radius.
• These redox reactions with H2SO4 have to be known.

  
• Acidified silver nitrate is used to test for halide ions as it reacts to form different coloured precipitates deepening on the ion present. If this isn't clear they can be tested further using ammonia.
• Cl- reacts with AgNO3 to form a white precipitate.
• Br- reacts with AgNO3 to form a Cream precipitate.
• I- reacts with AgNO3 to form a yellow precipitate.
• AgCl reacts with dilute ammonia and the precipitate dissolves.
• AgBr reacts with dilute ammonia and the precipitate remains.
• AgI reacts with dilute ammonia and the precipitate remains.
• AgCl reacts with concentrated ammonia and the precipitate dissolves.
• AgBr reacts with concentrated ammonia and the precipitate dissolves.
• AgI reacts with concentrated ammonia and the precipitate remains.
• Chlorine reacts with cold water to form chlorate (I) ions and chloride ions.

  
• The reaction of chlorine with cold water is known as a disproportionation reaction as the chlorine is both oxidised and reduced. The oxidation state goes from zero to both +1 and -1.
• In the presence of UV light, chlorine decomposes water to produce oxygen and hydrochloric acid. The chlorine is reduced in this reaction.

  
• Chlorine is used in small quantities to kill bacteria in water treatment processes.
• Anions- Sulfate 2- can be tested using barium chloride which reacts to form a white precipitate.
• Anions- Hydroxide 1- can be tested using red litmus paper which turns blue or universal indicator which turns blue-purple.
• Anions- Carbonate 2- can be tested for by adding an acid such as HCL, the substance containing the carbonate ion would then fizz and CO2 gas is given off. This gas can be collected and bubbled through limewater and if it turns cloudy, a positive result is confirmed.
• Group 2 cations can be identified via flame tests:
• Calcium cation can be identified if the flame turns brick red.
• Strontium cation can be identified if the flame turns red.
• Barium cation can be identified if the flame turns pale green.
• If ammonium ions are present it can be tested by holding red litmus paper over a Petri dish and if it turns blue ammonium ions are present.
• Halide ions have an increased reducing ability down group 7. This is because the size of the ion increases which means more electron shielding and weaker nuclear attraction. Thus the electron is more easily lost.
• The Halogens are less oxidising as you move down the group. This is demonstrated bu the reactions of sodium halides with sulphuric acid. The halogens can displace any halide which is below it in the periodic table.
• Reactivity decreases down group 7 due to more electron shielding and a larger atomic radius resulting in weaker nuclear attraction.
• When halogens react they gain an electron.
• Down group 7, electronegativity decreases due to increased atomic radius and electron shielding meaning electrons are less attracted to the larger atoms.
• Down group 7 boiling point increases.