Hydrogen Spectrum

Although hydrogen atom has only one electron, yet its spectrum consists of a large number of, lines. Bohr gave following explanation for this interesting problem.

In the ground state the single electron of a hydrogen atom keeps on rotating in the first energy level. But when energy is supplied to it (i.e. to gas), it is excited and its electron jumps to a higher energy level (2, 3, 4 or 5, etc.) by absorbing a photon of energy. Further since a given sample of hydrogen contains a very large number of atoms (almost infinite) and hence infinite electrons, different atoms absorb different amounts (quanta or photon) of energy. Hence the single electron in different atoms will jump to different energy levels depending upon the energy absorbed by the atom. For example, some of the hydrogen atoms may absorb energy to Jump directly from energy level 1 to 3, while still others may jump to higher energy levels 4, 5, 6 etc. by absorbing suitable energy.

The electrons then tend to fall back, almost immediately, to one or other of the lower energy levels in one or more jumps. For example, some electrons, say in energy level 4, may return directly to ground state (energy level l), others may first fall to energy level 3 and then to 1, while some others may first fall to energy level 2 and then to 1. This also happens to electrons excited to other energy levels, viz. 2, 3, 5, 6, 7 and 8. Thus, in general, different excited electrons adopt different routes to return to the ground state.

During each jump from a higher to a lower level, different amount of energy is released which appears in the form of a photon of light of specific frequency and thus gives a different line in the spectrum. Hence although hydrogen atom has only one electron, a number of lines (each corresponding to the energy of a photon released) appear in its atomic spectrum.

Spectral series

(i) Lines appearing in the atomic spectrum due to fall of electrons from higher energy levels (i.e. 2, 3, 4, 5, 6, etc.) to the lowest energy level, i.e. 1 were discovered by Lyman and hence named as Lyman series.

(ii) Lines appearing due to fall of electrons from energy levels 3, 4, 5, 6, 7, etc. to the energy level 2 were named as Balmer series.

(iii) Similarly, the Paschen, Brackett and Pfund series correspond to the fall of electrons from higher energy levels to energy levels 3, 4 and 5 respectively.