In salts of boron it exists as a cation in the B(III) form. It is known to impart green colour in several of its salts like borates, boron triflourides, boric acid etc. If there are no electrons in its valence shell to excite (in its cationic form) and hence obtain these characteristic colors, then what is responsible for this effect?
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4$\begingroup$ Ions are not responsible for the color. The color you are seeing is due to "atomic emission" resulting from the atomization of ions $\endgroup$– Nilay GhoshCommented Jul 16, 2021 at 7:45
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2$\begingroup$ @Ashish The salts don't necessarily have to be colorless but they will impart characteristic colors when in flame. In fact this can be used for qualitative analysis to check if and which alkaline metal is contained in your sample. Sodium for instance will give a bright yellow flame. See for instance i.ytimg.com/vi/9oYF-HxtoYg/maxresdefault.jpg $\endgroup$– RavenCommented Jul 16, 2021 at 8:14
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2$\begingroup$ @Poutnik. In his book, Herzberg says that the blue color of methane flame is due to $\ce{C2}$, as its vibrational structure is typical of $\ce{C2}$ $\endgroup$– MauriceCommented Jul 16, 2021 at 9:41
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2$\begingroup$ Don’t forget flame species such as CaOH, SrOH, etc. An earlier version of the publication below, freely available on the web, appeared in Journal of Pyrotechnics, No. 18, 2003. Color Values and Spectra of the Principal Emitters in Colored Flames, W. Meyerriecks* and K. L. Kosanke. Boron is not discussed in the paper, but a few other common elements are discussed. $\endgroup$– Ed VCommented Jul 16, 2021 at 13:28
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2$\begingroup$ And that such boron cations don't exist in any compound or even any flame, is no problem? One would truly extreme temperatures to triply ionise boron, or anything else. $\endgroup$– MithoronCommented Jul 17, 2021 at 23:45
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Whenever you see low temperature colored emission in flames, it is invariably because of the molecular species, do not attribute all flame colors due to atomic emissions right away (alkali metals are exceptions). This beautiful green color of boron flames is due to a small molecules containing boron. The green band centered at 547 nm has puzzled spectroscopists for a while. It has been attributed to a radical BO$_2$ and people have excluded B$_2$O$_3$. See for example
Kaskan, W. E., and R. C. Millikan. "Source of Green Bands from Boron‐Containing Flames." The Journal of Chemical Physics 32.4 (1960): 1273-1274.
There are two requirements in order to see the green color of boron-flames.
(i) You must have a lot of "OH" entities in the flame, which means an oxygen rich is good.
(ii) Boron must be present too (obviously!)
Even elemental boron when burnt properly can generate this characteristic color which is often seen with boron and alcohol solutions.
Now an interesting question: what do we expect from boron's atomic emission? It indeed has valence electrons. The strongest emission is in the UV range which your eyes can never see! Around 200 nm. This is the proof that boron "atoms" are not emitting this green color. You can see this in Grotrian diagram which simply shows what wavelengths (Angstrom units) to expect in an atomic emission. Bor III means neutral boron atom (in German). This is from a old book by Grotrian, " Graphische Darstellung der Spektren von Atomen und Ionen mit ein, zwei und drei Valenzelektronen: " Freely available from Google books.
