The idea of electron shells was developed by Niels Bohr in his 1913 model of the atom, which places electrons into stable orbits around the nucleus. The Bohr model was a modification of the Rutherford model, which did not require that electrons move in orbits of fixed size and energy.
In the Bohr model, the energy of each electron shell is related to shell size, with larger shells containing more energy. When electrons move from one shell to another, they absorb or emit energy in the form of radiation.
The outermost electron shell of an atom is called the valence shell. The electrons in this shell, known as valence electrons, are the primary particles involved in chemical reactions. Valence electrons between atoms interact with each other to form chemical bonds.