The Bohr model is a simple model of atomic structure. It is now outdated, but is still useful at the basic level. An atom has a central core called its nucleus which contains its protons (positively charged particles) and neutrons (neutral particles). The Bohr model depicts electrons, tiny negatively charged particles, as orbiting the nucleus in concentric rings, representing energy levels. However, the electrons reside in atomic orbitals, which are more complicated.