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Topic: inorganic compounds  (Read 1965 times)

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Offline annalisa23061999

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inorganic compounds
« on: February 22, 2018, 11:16:27 AM »
why does
Li+    -> color red
Na+  -> color orange
K+     ->  color lilac
Cu+2  ->  color green

change into these colours when heat up by the flame?
is it because of their shells, i cant work it out

Offline mjc123

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Re: inorganic compounds
« Reply #1 on: February 22, 2018, 12:49:35 PM »
The flame colours of ions are not easy to predict. It does depend on their "shells", i.e. the available electronic energy levels, and the allowed transitions between them, and which are in the visible range. The visible spectrum covers a quite small part of the whole range of possible transition energies (have you heard of the Lyman, Balmer, Paschen etc. series of lines in the hydrogen spectrum, for example?), and in a sense it's a bit "random" where in the visible range a visible transition for a particular element will come, in the sense that there are no obvious periodic trends in colour.

Offline Corribus

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Re: inorganic compounds
« Reply #2 on: February 22, 2018, 03:31:11 PM »
Not to mention (Ok I'm mentioning it), extrapolating a color perceived by your eye to a complex electromagnetic emission spectrum is not straightforward, either.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline annalisa23061999

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Re: inorganic compounds
« Reply #3 on: February 23, 2018, 04:23:54 AM »
thank you so much for taking the time to help me.

Offline Enthalpy

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Re: inorganic compounds
« Reply #4 on: February 23, 2018, 12:13:38 PM »
Do ions emit the light in this flame test, or rather neutral atoms?

I doubt about ions because
- Orange for Na is the same colour as Na lamps, where atoms are said to emit the light. 555, 589nm are lines of the neutral atom.
- Ions are scarce in a flame, but atoms of volatile elements are plentiful.
- Neutral Na or K are rather easy to excite while Na+ or K+ need a big energy, and their low transitions are in the UV.

Offline Corribus

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Re: inorganic compounds
« Reply #5 on: February 27, 2018, 10:22:54 AM »
In ICP-OES you get emission lines from both atoms and ions, depending on how easy it is to ionize atoms in each element. But the argon plasma is in the neighborhood of 104 K. A flame temperature is much lower (for Bunsen burner, about 1770 K), and maybe not enough to ionize even easily ionizable elements. Quick math supports this.

The first ionization energy of sodium is 5.139 eV, or about 8.2 x 10-19 J. For a Bunsen flame at 1770 K, kT ~ 2.4 x 10-20 J. For an ICP plasma at 10,000 K, kT ~ 1.4 x 10-19 J. The ICP plasma is much more likely to result in sodium ion emission lines because kT is much closer to the ionization energy. In fact an ionization buffer (usually cesium) is added during ICP-OES experiments to reduce ionization, which can skew measurements.

Potassium is easier to ionize than sodium, and cesium easier to ionize than potassium. So even in a Bunsen flame you may start to see some emission from ions as you go down columns in the periodic table. Still, the flame temperature is pretty low, so even for cesium (ionization energy ~ 3.9 eV, 6.2 x 10-19 J) atomic emission will likely dominate.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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