The Bohr model of carbon has a central nucleus containing six protons and six neutrons, encircled by an inner orbit of two electrons and an outer orbit of four electrons. The two orbits represent different energy levels and are at a set distance from one another and from the nucleus.
In the Bohr model, electrons with less energy occupy orbits closer to the nucleus and are uninvolved in chemical bonding. When an electron receives a packet or quantum of energy, it leaps to the next orbit. When an electron loses energy, it releases a quantum of energy and leaps down to the closer orbit. The outermost orbit is called the valence shell. In the case of carbon, the four electrons in the valence shell allow it to form molecules in a huge number of different ways.