The Rutherford–Bohr model is a model of the atom which assumes electrons can orbit in quantized levels, know as shells. For hydrogen, the binding energy of the nth shell is given by As such, the energy of a photon being absorbed or emitted by an atom which causes an electron to jump from nj to ni is where h is Planck's constant, c is the speed of light and λ is the wavelength of the photon. The ionization energy is the energy needed to completely remove an electron, or when ni = ∞. Assuming the atom is in its ground state (nj = 1), this is simply 13.6 eV. The above equation can be written as