Observatoire de Paris
<-   The hydrogen atom   ->

The "classical" model of the hydrogen atom is one electron orbiting one proton.

The electrons are located at specific distances from the nucleus. At rest (n=1), the orbit of the electron has a radius of 10^(-11)meters (Bohr classical radius). The electron can also be on larger orbits, associated with integers n = 2, 3, 4...n = ∞

The nucleus has a radius of 10^(-15) meters. The atom at rest is 10000 times bigger than the nucleus. If the nucleus was a coin of 10 centimes, the atom would be a sports ground.

An atom can pass from a fundamental state (n=1) to an excited one, absorbing a photon. It can also come back to the fundamental state emitting light, the colour of which (wavelength) depends on the energy levels of the atom.

The transition from a level n2 to a level n1 corresponds to an emission/absorption of wavelength lambda, such that 1/lambda=R*(1/n1^2 -1/n2^2), with R=1.1*10^(7) * m^(-1)


If the atom gets a sufficient amount of energy, the electron passes from the level n=1 to the level infinity. The atom loses its electron and becomes an ion. The corresponding wavelength is 0.91 *10^(-6)meters, i.e. in the UV range.

In the atmosphere of a star, the hydrogen atoms, enlightened by the star, only absorb the colours that make them pass from one level to another.