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Atoms are made up of subatomic particles such
as Electrons, Protons and Neutrons.
Of these protons and electrons are known as
“Nucleons”,as the nucleus of the atom is made
up of these particles.
The positive nuclear charge is neutralized by the
negatively charged electrons outside the nucleus.
The British physicist Ernest Rutherford, winner of
the 1908 Nobel Prize in chemistry, pioneered the field
of nuclear physics with his experimental research and
his development of the nuclear theory of atomic
The first atomic modal was proposed by
J.J.Thomson, who discovered electrons in 1903.
He however placed the electrons inside the
nucleus.Such a modal could not account for many
experimental observations.
In 1911,Rutherford, based on his x-rays scattering
experiment,proposed an alternate model & named it
is “Planetary model or nuclear model”
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In 1913,Danish scientist Niels Bohr put
forward a theory to overcome the detects in
Rutherford’s atomic model.
Electrons are moving with high velocity around
the nucleus in specified paths called “ orbits or
shell ”.As along as the electron is in a particular
orbit is its energy is constant.Therefore these
orbits are called “stationary orbit or stationary
shell ”.
Bohr proposed that ‘n’ can take values
1,2,3,4,…etc &named it is the
principal quantum number.
Each stationary orbit corresponds to a
definite energy.There stationary orbit
are designated by K,L,M,N,O,… .The
orbit close to the nucleus has less
energy compared to the orbit away from
the nucleus.
Atomic spectra display fine structure due to splitting
of spectral lines. I an attempt to account for the fine
structure, Arnold Sommerfeld proposed elliptical
orbits instead of circular orbits proposed by Bohr.
The atomic spectra I lines,in the
presence of applied magnetic field,
spit into several lines.This
phenomenon is called the ‘Zeeman
 In 1925 Uhlenbeck and Goudsmit
suggested that a moving electrons in an
orbit revolves or spins on its own axis.this
is similar to the earth revolving on its own
axis & also revolving around the sun..
 Such a spin would contribute to the
angular momentum of the electron.The
electron can spin clockwise or anti
clockwise and this is represent as and