Rutherford Atomic Model

Lord Rutherford in 1911 carried out series of experiments. He passed a beam of ∞-particles through a very thin gold metal foil. He found that most of the particles passed through it without any deflection. However some of them deflected at large angles and very few of them bounced back.

From the result of his experiment, Rutherford concluded that:

1.    The volume occupied by an atom must be largely empty as most of the particles passed through the foil un-deflected.

2.    The positive charge, in the atom is concentrated in extremely dense region which he called the nucleus. This was from the fact that particles after collision with a heavy positively charged nucleus had bounced back.

From the above observations, Rutherford proposed that the atom consists of very small, positively charged nucleus in which the most of the mass of the atom is concentrated. The rest of the volume is empty space, however this space is not completely empty and that in it electrons revolve around the nucleus.

According to the Rutherford model, an atom consisted of two parts.

1.    Nucleus.

2.    Extra nuclear part.

The proton and neutron reside in the nucleus. Since the protons are positive charged particles; therefore, the nucleus has positive charge. Further since the weight of the atom due to presence of protons and neutrons, as these particles are residing in-the nucleus, the weight of the atom is concentrated in the nucleus.

The electrons are revolving around the nucleus in the extra nuclear part in various orbits which are also called as shells, or energy levels.

3.3.1 Weaknesses or Defects in Rutherford's Atomic Model:

1.    According to the classical physics, since electron is revolving around the nucleus constantly it should lose energy and it ultimately falls into the nucleus.

2.    If the revolving electron emits energy continuously, then there would be a continuous spectrum, but in contrast to it, we get line spectrum from the atoms of element.

3.3.2 Neil Bohr's Atomic Model:

After Planks and Einstein's .discoveries, Niel Bohr, a Danish Physicist in 1913 offered a theoretical explanation of line spectra. -The important assumptions for the atomic structure are given below:

1.    Neil Bohr adopted Planks idea, that energies are quantized. He proposed that the electrons in atoms move only in certain allowed energy levels, (energy states), so an electron in an allowed energy state will not radiate energy continuously and therefore will not fall in the nucleus.

2.    That the atom radiates energy as a light only when the electron jumps from higher energy level (E2) to lower energy level (E1). The quantity of energy radiated is in discrete quantity, called quanta. A quantum of energy is directly proportional to the frequency of the radiation.

i.e.      ∆E = E2-E1=hv

Where h = Planks constant

          V = is the frequency of the radiation.


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