We now address the spectral lines and the information contained in the spectral lines. The lines thus served as the fingerprints of the atoms. To understand why this is true we need to discuss the structure of atoms for a little while.
Atoms are primarily empty space.
That's a lot. Now, back to atomic structure. If we make an analogy with the Solar System , we can imagine that the nucleus is equivalent to the Sun, the electrons are like the planets, and the electrical force plays the role of gravity. There is, however, a huge difference between how our Solar System works and how an atom works. In our Solar System, the planets, in principle, can move about the Sun in orbits of arbitrary size.