In 1915, Niels Bohr proposed the Bohr model, which is a (mildly incorrect) simplified version of quantum mechanics.
His "planetary model" included protons as the nucleus, which was overall positively charged. This nucleus occupied a central region of the atom, and the electrons orbit the nucleus. He proposed that electrons existed at set levels of energy, and therefore at fixed distances from the nucleus.
His "planetary model" included protons as the nucleus, which was overall positively charged. This nucleus occupied a central region of the atom, and the electrons orbit the nucleus. He proposed that electrons existed at set levels of energy, and therefore at fixed distances from the nucleus.