Bohrs Atomic Model

Md. Saifur Rahman

Bohrs Atomic Model

Scientist Neils Bohr gave some corrections to Rutherford's atomic model in 1913. This corrected model is called the Bohr's atomic model. The main postulates of this model are -

a) The electron moves around the nucleus in a circular motion, about an axis. This circular motion is called an energy level or orbit. They are also known as the shell or permanent orbit or principal energy level. Electrons do not absorb or emit any energy when they revolve these fixed orbits. If we express the permanent orbit as n, then n = 1, 2, 3, 4, etc.

In other words, if n = 1, the principal energy level is K,

when n = 2, L is the principal energy level,

when n = 3, M is the principal energy level,

when n = 4, N is the principal energy level, etc.

b) According to Bohr's model, the angular momentum of an electron in an energy level is mvr = ^nh/_2𝜋.

Here,

m = the mass of the electron (9.11×10^-31 kg)

r = the radius of the orbit that the electron follows

v = the velocity of the electron in the permanent orbit

h = Plank constant (h = 6.626×10^-34 m² kg/s)

n = principal energy level or principal quantum number (n = 1, 2, 3, 4, .......)

According to this, the energy level with a lesser value of n is the lower energy level and the energy level with a higher value of n is the higher energy level.

c) When an electron moves a round in its principal energy level, it doesn't absorb or emit any energy. However, it absorbs energy when it moves from a lower to a higher energy level. In the same way, an electron emits energy when it moves from a higher to a lower energy level.

This absorbed or emitted energy, E = hv = ^hc/_λ.

Here,

c = velocity of light (3×10⁸ ms^-1)

v = frequency of absorbed or emitted energy (unit s^-1 or Hz)

λ = length of energy wave absorbed or emitted (unit m)

The light emitted when an electron moves from higher energy level to lower energy level creates an atomic spectra when it goes through a prism.

The success of Bohr's Model

a) Rutherford compared the orbits of electrons around nucleus to that of the planets in the solar system. It did not mention anything about the size of the energy levels. Bohr's atomic model specifies the circular size of the energy levels.

b) Rutherford's model does not mentioned about the changes in structure of atom when they absorb or emit energy. The Bohr model states that the electron moves from lower to upper energy level when the atom absorbs energy and it moves from upper to lower energy level when the atom emits energy.

c) The Rutherford model doesn't explain the atomic spectra but the Bohr model can explain the atomic spectra of hydrogen (H) atom with one electron.

Limitations of Bohr's Model

a) Although the Bohr model can explain the spectrum of hydrogen-containing one electron, it cannot explain the spectrum of ions or atoms containing more than one electron.

b) When electrons are transferred from one energy level to another, according to Bohr, there will be a single line in the spectrum. But highly sensitive apparatus shows every line consists of several lines. Why each line is the summation of the various lines is not explained.

c) The Bohr model specifies that orbits of electrons in an atom are circular. However, later on, it was proved that the orbits can be of oval shape too.