Leucippus and Democritus were the first to propose, in the fifth century B.C., that all matter is made of tiny units called atoms. The two philosophers held that these were solid particles without internal structure, and came in a variety of shapes and sizes. Intangible qualities such as taste and color, according to this theory, were made of atoms. However, Aristotle strongly opposed this idea, and the scientific community failed to pay serious attention to it for centuries.

1. In an attempt to account for the structure of line spectra, Sommerfeld modified Bohr’s atomic model by adding elliptical orbits.
2. While retaining the first of Bohr’s circular orbit as such, he added one elliptical orbit to Bohr’s second orbit, two elliptical orbits to Bohr’s third orbit etc.
3. Nucleus of the atom is one of the principal foci of these elliptical orbits because periodic motion under the influence of a central force will lead to elliptical orbits with the force situated at one of the foci.
   Merit: Bohr - Sommerfeld model successful in accounting for the fine line structure of hydrogen atomic spectra.
   Limitations :
4. This model failed to account for the atomic spectra of atoms of more than one electron.
5. It did not explain Zeeman and stark effects.

Bohr’s Model of an Atom :

Niels Bohr proposed that electrons in an atom occupy ‘stationary’ orbits (states) of fixed energy at different distances from the nucleus.
When an electron ’jumps’ from a lower energy state (ground state) to higher energy states (excited state) it absorbs energy or emits energy when such a jump occurs from a higher energy state to a lower energy state.
The energies of an electron in an atom can have only certain values E₁, E₂, E₃; that is, the energy is quantized. The states corresponding to these energies are called stationary states and the possible values of the energy are called energy levels.