Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Halide Ion
What is Halide Ion?
The negative ion formed when a halogen atom gains one electron, giving it a full outer shell and a charge of one minus. Chloride, bromide and iodide ions are single ions written as Cl minus, Br minus and I minus, and in aqueous solution they are colourless, unlike the halogens themselves.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Halide Ion in context
Down Group I the alkali metals become more reactive and lower-melting as the single outer electron is lost more easily, while down Group VII the halogens become less reactive as it becomes harder to gain the extra electron needed to fill the outer shell. A more reactive halogen displaces the halide ion of a less reactive one from solution. Transition elements, between Groups II and III, show variable oxidation numbers rather than one fixed charge, and Group VIII noble gases are unreactive because their outer shell is already full.
Common mistakes with Halide Ion
- Error 14 · “Potassium bromide solution is red-brown.” Defect The colour of the element attributed to its ion. Chemistry Halide ions in solution are colourless. Red-brown is the free element bromine. This matters because the colourless starting solution is what makes a displacement visible. Repair Halide solution \(=\) colourless. Halogen in solution \(=\) coloured. Transfer What colour is aqueous potassium iodide? Colourless.