Spectral lines required atom so only certain amounts of energy could be immitted or absorbed. All atoms have a nucleus made up of: protons (positive electric charge, neutrons (neutral no electric charge, electrons (neg electric charge) orbit nucleus. Protons and neutrons have same mass but electron 2000 times less massive. Hydrogen simplest and most common atom in universe: nucleus of one proton with electron orbiting it. Ground state: electron has lowest possible energy and orbits close to nucleus, electron in this state can absorb energy from radiation and move into excited state, move back into ground state and re-emit radiation. Excited state: electron has higher energy than ground stat orbits farther from nucleus. Emission energies correspond to energy differences between allowed levels. Bohr model: electron orbits proton in well defined orbit wrong, electron moving in circle means its accelerating (moving direction, accelerating electric charge produces electromagnetic radition, electrons would radiate all their energy away and spiral into nucleus collapse.