The number of protons in the nucleus determines the type of element that the atom represents. The Bohr model of the atom was an early attempt to explain how atoms can produce emission and absorption line spectra. If an orbiting electron is given enough energy, it can escape from the atom, which is then said to be ionized . Between these two states, the electron can exist only in certain well-defined states each with a very specific energy. The electron's energy is said to be quantized .
Electromagnetic radiation exhibits both wave and particle properties.
Particles of radiation are called photons . The energy of a photon depends on its color-it is directly proportional to the photon's frequency. As electrons move between energy levels within an atom, the difference in the energy between the states is emitted or absorbed in the form of photons. Because the energy levels have definite energies, the photons also have definite energies, and hence colors, which are characteristic of the type of atom involved. Molecules are groups of two or more atoms bound together by electromagnetic forces.