Flame colors are produced from the movement of the electrons in the metal ions present in the compounds.
For example, a sodium ion in an unexcited state has the structure 1s2 2s2 2p6. When you heat it, the electrons gain energy and can jump into any of the empty orbitals at higher levels - for example, into the 7s or 6p or 4d or whatever, depending on how much energy a particular electron happens to absorb from the flame.
Because the electrons are now at a higher and more energetically unstable level, they tend to fall back down to where they were before - but not necessarily all in one go.
An electron which had been excited from the 2p level to an orbital in the 7 level, for example, might jump back to the 2p level in one go. That would release a certain amount of energy which would be seen as light of a particular color.
However, it might jump back in two (or more) stages. For example, first to the 5 level and then back to the 2 level.
Each of these jumps involves a specific amount of energy being released as light energy, and each corresponds to a particular color.
As a result of all these jumps, a spectrum of colored lines will be produced. The color you see will be a combination of all these individual colors.
The exact sizes of the possible jumps in energy terms vary from one metal ion to another. That means that each different ion will have a different pattern of spectral lines, and so a different flame color.
The colors
The colors in the table are just a guide. Almost everybody sees and describes colors differently. I have, for example, used the word "red" several times to describe colors which can be quite different from each other. Other people use words like "carmine" or "crimson" or "scarlet", but not everyone knows the differences between these words - particularly if their
Li red
Na strong persistent orange
K lilac (pink)
Rb red (reddish-violet)
Cs blue? violet? (see below)
What do you do if you have a red flame colour for an unknown compound and don't know which of the various reds it is?
Get samples of known lithium, strontium (etc) compounds and repeat the flame test, comparing the colours produced by one of the known compounds and the unknown compound side by side until you have a good match.
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Note: I don't have confidence in the caesium flame colour. There is total disagreement about this on the web and in the books I have looked at, and I have never seen this flame colour myself. However, I have received a helpful email from a student who says: "At my school we did some flame testing experiments, and . . . caesium is actually either blue or violet, depending on the way you look at it. I think it looks more violet than blue, but it sort of changes each time you do it." (Kara Gates, March 2006). If you thought chemistry was clear-cut, you are sadly mistaken!
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For example, a sodium ion in an unexcited state has the structure 1s2 2s2 2p6. When you heat it, the electrons gain energy and can jump into any of the empty orbitals at higher levels - for example, into the 7s or 6p or 4d or whatever, depending on how much energy a particular electron happens to absorb from the flame.
Because the electrons are now at a higher and more energetically unstable level, they tend to fall back down to where they were before - but not necessarily all in one go.
An electron which had been excited from the 2p level to an orbital in the 7 level, for example, might jump back to the 2p level in one go. That would release a certain amount of energy which would be seen as light of a particular color.
However, it might jump back in two (or more) stages. For example, first to the 5 level and then back to the 2 level.
Each of these jumps involves a specific amount of energy being released as light energy, and each corresponds to a particular color.
As a result of all these jumps, a spectrum of colored lines will be produced. The color you see will be a combination of all these individual colors.
The exact sizes of the possible jumps in energy terms vary from one metal ion to another. That means that each different ion will have a different pattern of spectral lines, and so a different flame color.
The colors
The colors in the table are just a guide. Almost everybody sees and describes colors differently. I have, for example, used the word "red" several times to describe colors which can be quite different from each other. Other people use words like "carmine" or "crimson" or "scarlet", but not everyone knows the differences between these words - particularly if their
Li red
Na strong persistent orange
K lilac (pink)
Rb red (reddish-violet)
Cs blue? violet? (see below)
What do you do if you have a red flame colour for an unknown compound and don't know which of the various reds it is?
Get samples of known lithium, strontium (etc) compounds and repeat the flame test, comparing the colours produced by one of the known compounds and the unknown compound side by side until you have a good match.
________________________________________
Note: I don't have confidence in the caesium flame colour. There is total disagreement about this on the web and in the books I have looked at, and I have never seen this flame colour myself. However, I have received a helpful email from a student who says: "At my school we did some flame testing experiments, and . . . caesium is actually either blue or violet, depending on the way you look at it. I think it looks more violet than blue, but it sort of changes each time you do it." (Kara Gates, March 2006). If you thought chemistry was clear-cut, you are sadly mistaken!
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