The hydrogen atom

Hydrogen is the simplest and most abundant element in the Universe.

The “classical” model of the hydrogen atom is an electron orbiting around the nucleus, consisting of a single proton ^? .

In the atoms, electrons are at precise distances from the nucleus. In the ground state ($n=1$), the radius of the electron’s orbit is $10^{-11}$ meters (classical Bohr radius). The electron can also follow larger orbits, associated with integers $n = 2, \space 3, \space 4 \space \cdots, \space n = \infty$.

The nucleus has a radius of $10^{-15}$ meters. The atom in the ground state is thus 10,000 times larger than the nucleus. If the nucleus was the size of a ten cent piece, the atom would be as big as a football field.

The atom can pass from the ground state ($n=1$) to an excited state by absorbing a photon ^? of light. It can also return from an excited state to its ground state by emitting light, the colour of which (wavelength) will depend on the atom’s energy levels.

Passing from level $n^2$ to level $n^1$ corresponds to the emission/absorption of wavelength λ, such that

$$ \frac {1} {\lambda} = R ( \frac {1} {n_1^{2}} -\frac {1} {n_2^{2}}) $$

With $R = 1.1 \times 10^7 {\text{m}}^{-1}$.

If the atom receives enough energy, the electron goes from level $n = 1$ to level $n = \infty$. The atom loses its electron and becomes an ion. The corresponding wavelength is $0.91 \times 10^{-6}\text{m}$ which corresponds to ultra-violet.

In the outer shell of a star, the hydrogen atoms illuminated by the star only absorb the colours which cause them to pass from one level to another. This mechanism defines the spectrum lines that are to be found in the star’s spectrum.