Niels Bohr was a physicist whose genius provided the first quantum model of the atom. Before Bohr, the 'planetary' model of the atom was unstable. Bohr proposed that electrons move in fixed orbits around the nucleus and can jump between these orbits by absorbing or emitting specific amounts of energy (quanta). This explained why elements emit light in specific colors (spectral lines). He also formulated the 'Principle of Complementarity,' arguing that objects can have dual properties—like light being both a wave and a particle—depending on how they are measured. Bohr’s institute in Copenhagen became the world center for quantum physics, and he was a key figure in the debate over the philosophical meaning of quantum mechanics.