Can a flame test be used to identify a metal atom in a compound? why or why not? what about a nonmetal atom?

Flame test can be very beneficial in identifying the metal ions in compounds.
This is because each metal ions gives a special color to the flame. This color is used to identify the metal ion (check the attached image which shows the fame color that is produced from each metal).

This occurs because when the compound is heated, electrons of the metal ion gain energy and can jump to a higher energy level where they become unstable. Due to this non-stability, the electrons return back to their original level releasing the gained energy in the form of a light. Since these transitions are different from one metal to another, the light released from each metal has a specific color that characterizes this metal.

However, the flame test is more useful for some metals that the others, especially those of the first group.

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Outermost (valence) electrons