Question

how can you tell whether an electron is in an excited state

Answer 1

You would need special machines to tell this. They are in excited state for a very small amount of time.

Answer 2

first, electrons don't stay in the excited state for very long, it's miliseconds or less. if an electron was in an excited state, when it falls back to a lower state or its ground state, it emits light energy.

When an atom enters the excited state, an electron moves up to a higher energy level, and releases energy. An electron in the excited state is not stable until it returns to ground state.

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