Group 7 Elements: Uses and Halide Tests

Properties of Group 7 Elements and Compounds Group 7 elements become less reactive down the group Group 7 elements react with metals to form ionic halides with an X- ion FluorinePale yellow-green gas, rarely seen as an element because it is so reactive. Reacts with almost anything.HalidesIonic halides typically have a halide ion with a 1- charge. In contrast to the reactive elements, halide compounds are mainly very stable. Sodium chloride  NaCl - common salt Sodium fluoride NaF and tin (II) fluoride SnF2 are fluoride compounds added to toothpaste to prevent tooth decay Crystals of calcium fluoride, CaF2, also known as fluoride and fluorspar, are used to make lenses to focus infrared light. Testing for Halide IonsThe presence of halide ions can be detected with a simple tube test. The unknown halide sustance is first dissolved in water. An aqueous solution of silver nitrate, AgNO3 is added. Silver ions, Ag+ from the AgNO3 react with any halide ions, X- present, forming a silver halide precipitate, AgX The silver halide precipitates are coloured - the colour tells us which halide is present. Sometimes it is difficult to judge the exact colour. if you are unsure, add aqueous ammonia, NH3 different halide precipitates have different solubilites in aqueous ammonia - this confirms which halide is present. Halide test results are: chloride: Ag+ + Cl- ---> AgCl   white precipitate, soluble in dilute NH3 bromide: Ag+ + Br- ----> AgBr  cream precipitate, soluble in conc NH3 iodide: Ag+ + I- ------> AgI        yellow precipitate, insoluble in conc NH3 This type of reaction is called a precipitation reaction. A precipitation reaction takes place in aqueous solution when aqueous ions react together to form a solid precipitate.

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