Niels Bohr, the 20th-century Nobel laureate Danish physicist, discovered and developed a realistic model of the atom, known as the Bohr model. Additionally, he developed the principle in quantum mechanics known as complementarity.
In the Bohr atomic model, a small nucleus of protons and neutrons is surrounded by electrons that maintain separate orbits, repelled by electronegativity. The electrons in this model can "jump" from one orbit to another when interacting with other energy sources.
The Bohr model was a combination of the Rutherford model of the atomic nucleus and Planck's quantum theories. It was largely supplanted over time by other models of atomic behavior, though it is used as a simple and fundamental description.