Bohr Model and Spectral Lines

In Model mode, the atom is shown in two ways side by side: the Bohr model, with the K, L, M… shells, and the energy ladder by subshells (s, p, d). When the electron jumps from a higher level to a lower one, it emits a photon whose color depends on the energy difference between the levels. In Experiment mode, the light from a discharge tube passes through a slit and a prism and forms, on the photographic plate, the spectral lines of the element: a "barcode" of light that identifies each atom. With the absorption spectrum, white light passes through the atom (in the Experiment, a lamp shines on the gas in the tube, with no discharge): it only absorbs the colors with the exact energy of a jump from the ground level, which become dark lines. That is why the absorption spectrum has fewer lines than the emission spectrum. Many gases, such as hydrogen and helium, only absorb ultraviolet: on the photographic plate, which shows only the visible, no dark lines appear. In Build mode, the atom is built by dragging protons, neutrons and electrons (or by choosing the element in the periodic table), up to oganesson (Z = 118), with the configuration given by the Aufbau (Madelung) diagram: the number of protons defines the element, extra or missing electrons form an ion, and neutrons change the mass (the isotopes). With the atom neutral, the "See the spectrum" button takes you to Model mode with that element (the simulator only has the lines of neutral atoms).

The energy levels and the lines are measured values, taken from the NIST atomic spectra database. The color of each line comes from the energy difference between two levels, $\lambda = hc/\Delta E$. Only for hydrogen does the Bohr model give these values exactly ($E_n = -13{,}6/n^2$ eV); for the other atoms, the shell radii are only illustrative. Lines that are very close together appear as one. In absorption, the chance of each jump absorbing comes from the oscillator strength calculated with the NIST Einstein coefficients; the white light of the simulation sends more photons at the energies of these jumps than a real lamp does, so that the lines appear within seconds.

Marco P. M. de Souza