basic structure of the atom

Let's start with the basic structure of the atom. Every atom has a nucleus that contains protons which are positively charged with negatively charged electrons in orbitals around the nucleus. The number of protons is equal to the number of electrons so that the atom overall is neutral. Now electrons will be in the lowest energy orbital that is possible to be stable. This will be the orbital closest to the nucleus. Let's take lithium as an example. Lithium is the third element in the periodic table and has three protons and three electrons. Two electrons are in an orbital close to the nucleus. This orbital can only hold two electrons as it is has a small radius. So the third electron goes in an orbital a bit farther away from the nucleus. We say that lithium has an electronic configuration of 2,1. An electronic configuration is just a way of saying where the electrons are!

Now there are lots more orbitals further away from the nucleus that are empty in the case of lithium. They are higher in energy as they are further from the pull of the nucleus so an electron would need more energy to stay in those orbitals. If we give that one lone electron some energy, for instance in the form of heat, then it can jump from it's orbital into a higher orbital. Once there it is unstable and falls back to its original starting point. However, when it falls back it has to lose that extra energy and it does this in the form of light. The light is emitted and the wavelength of the light depends on the difference between the energies of the starting and finishing orbitals. These will be different for different metals so the wavelength of the light will be different.