The Bohr model


In 1913 Niels Bohr suggested that the nucleus could be like a mini solar system
with the electrons orbiting the nucleus. This fits in very nicely with what we know
about circular motion, since the centripetal force would be provided by the electric
attraction between the electron and nucleus. The problem with this model is that
as the electrons accelerate around the nucleus, they would continually radiate
electromagnetic radiation; they would therefore lose energy causing them to spiral
into the nucleus. This model therefore cannot be correct. One thing that the Bohr
model did come close to explaining was the line spectrum for hydrogen.


This model has the following postulates about atoms:-





  • The electrons are arranged in a series of energy levels or orbits which are located at various

      • precise distances away from the nucleus. Each of the energy levels has electrons with definite
        and fixed values of energy.

        • These energy levels correspond to the shells and sub-shells. Each energy level contains
        electrons with the same value of potential energy.

        • The energies of electrons are quantized. This means that an electron moving in an energy
        level or orbit can only have a certain amount of energy.

        • The electron nearest the nucleus has the lowest energy level (n = 1). The further the electron
        is from the nucleus, the higher its energy.

        • The integer n is known as the principal quantum number; n can have values from 1 to infinity (∞).
        • A ground state atom is one in which the total energy of the electrons cannot be lowered by
        transferring one or more of them to different orbits. That is, in a ground state atom all the
        electrons are in the lowest possible energy levels.