Bohr had given his atomic model which
was called the
Bohr’s atomic model, according to which 
the entire mass
of the atom was concentrated in the
nucleus.
Because the mass of electrons is negligible
as compared to the mass of protons.
So the entire mass of the atom is
present in the nucleus,
as the protons are present in
the nucleus of the atom.
Scientists took 2 nuclei- the
hydrogen nucleus
and the helium nucleus. So we know that
the entire mass
of an atom is concentrated in the
nucleus. So hydrogen has
1 proton, helium has 2 protons so the mass of
the mass of helium should be twice the
mass of hydrogen,
as helium has 2 protons and hydrogen
has 1 proton.
But when experiments were performed it was
observed
that the mass of helium was four times
the mass of hydrogen. So where was this extra
mass coming from
in the helium atom?
Scientists concluded that there should
be some extra particle
which is present in the nucleus of the
helium atom.
This extra particle is giving the extra 
mass
of the helium nucleus and this particle
should be neutral.
Because the atom is electrically
neutral, the number of electrons
is equal to the number of protons. So 
now if this particle has some 
charge
it will make the atom unstable. That will, that
is
it would not be electrically neutral.
Therefore, this
extra particle should be neutral in
nature.
One such scientist was James Chadwick.
He was interested and curious to know
where that extra mass in the helium nucleus was
coming from?
So he performed a number of experiments.
In one such experiment,
he used a beryllium nucleus and he projected
an alpha particle on this
beryllium nucleus.
So when he projected, he observed that 
a particle was released.
He called it neutron. Now he wanted to
check
if this neutron was neutral in nature
or not.
So he took this field.
This is a field of positive particles.
It has all the positive particles in it.
So if a positive particle is passed,
through this field, you see that
like
charges repel. So if a proton is passed
through this field, it is repelled by
these positive charges.
Then he passed the neutron the particle
that he had
achieved from the reaction.
He passed the neutron through this field.
He observed that the neutron was neither
attracted
nor repelled by these positive charges.
And so he concluded
that neutron is neutral in nature it has
no charger in it.
And also later by some experiments it was
shown that the mass of neutron
is equal to the mass of proton. Hence
the mystery of the mass of helium
nucleus was now solved.
Helium nucleus has 4 times
the mass of hydrogen nucleus. Therefore,
it has
2 extra neutrons. These neutrons
the 2 neutrons, make the mass of helium nucleus
4 times
the mass of hydrogen. Since we know that 
the mass of neutron
is equal to the mass of proton and
hydrogen nucleus
has only 1 proton. Therefore, these 4 particles,
2 protons and 2 neutrons, make the mass of helium
four times the mass of hydrogen and
hence, this is how
neutrons were discovered.
So this is the present accepted form of the
structure of atom.
The nucleus of the atom contains protons
and neutrons and the electrons revolve
around the nucleus in definite orbits. This
is the structure of atom as we know
today.
The credit for the discovery of neutron
goes to? So we know it was James Chadwick,
who had performed an experiment using
beryllium nucleus.
He had projected the alpha particles,
and then he had found out that
a particle neutron was released,
which was neutral in nature.
The credit for the discovery of the
neutron goes to James Chadwick.
