Why do magnesium ions give no flame colour?
Well , I’m not sure what you mean by putting “magnesium ion” in a flame, because like any ion, you can’t physically hold an ion. Just like salt breaking up into Na+ and Cl- ions, you can’t get a cupful of sodium ions but you can get a cup od Na powdered metal. Now if you are talking of putting a piece of Mg ribbon into a flame it emits a very strong white light which contains a broad spectrum of visible and invisible light. It emits infrared that you can’t see and it emits UV, as you pointed out, and you can’t see that. But you definitely can see the tremendous visible light range which is why they put powdered Mg in fireworks just for it’s brightness of flame. But now thinking about it, Mg burns brightly in open air. But maybe burning it in a flame excludes the oxygen from accessing the Mg and turning it into MgO. No oxygen exists within the flame of a burner. So it sounds like the flame actually boils the Mg and in the flame the solid Mg particles ionize into Mg gas. Yeah that would
