I wanted to show you some electrons but I was not allowed to do this. That's because you don't know yet if the electron is a particle or a wave. However, I will try to describe them for you. These electrons look the same, behave the same, you never know if one is yours or it belongs to a mate. For our purposes you can imagine the electron as being a little blue particle which travels around a nucleus.
All electrons are lazy - they always try to have as little energy as possible. If you ask them what orbital they prefer, they answer: "The orbital having the lowest possible energy". As a consequence of their laziness, my exterior shells are never occupied. My last shell containing some electrons is called the valence shell.
In atomic spectrometry you must fool the electrons living in the valence shell to jump from one orbital to another. This is not an easy job, because they don't make this jump if you don't offer them the required energy. The amount of energy required for this jump is the exact difference between the energies of the involved orbitals. The electrons won't accept more or less energy than this.
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