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
The volume occupied by an atom must be largely empty as
most of the particles passed through the foil
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
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
According to the classical physics, since electron is
revolving around the nucleus constantly it should lose
energy and it ultimately falls into the nucleus.
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
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:
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.
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.