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2-6: What is the Atomic Nucleus? |
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As discussed
in the preceding page,
2-5,
the success of
the Rutherford model
of the nuclear atom
taught us that
an atom consists
of the positively
charged massive nucleus
and the surrounding
light electrons.
An atom contains
Z electrons
whose total charge is
-Ze,
so that the positive charge
+Ze cancelling
this negative charge
of the electrons
is considered
to be concentrated
into the nucleus.
It has been explained
in detail
in the previous pages
that
the total mass
of the electrons
in an atom is
extremely smaller
than the total mass
of the atom.
This means that
almost all mass of an atom
is carried by the nucleus.
Then,
what is an atomic
nucleus made of?
What are the constituents
of the nucleus?
What is the structure
of the nucleus?
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[The Size of a Nucleus]
Rutherford's formula
of angular distribution
shown in the preceding page
is obtained
by assuming that
the charge of the nucleus
is concentrated
on a point,
i.e. a point charge.
However, even if the charge
spreads over an area
narrower than the minimum of
the closest approach
shown in the following
figure,
Rutherford's formula
must still hold.
Therefore, if the experimental
data are well reproduced
by Rutherford's formula,
the nuclear radius
of that atom
is understood to be
smaller than
the minimum of
the closest approach.
Let us look back
the trajectories
of Rutherford scattering
shown in
the following figure,
where the red arrow
at the central part
denotes the minimum of
the closest approach.
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In the experiment
of the alpha particle
scattering
for the target
of a copper foil,
the resultant data
are well fit
to Rutherford's
formula up to
the scattering angle
The incident energy
in this case is
E = 5.3 MeV,
and the atomic number
of copper is
Z = 29.
The minimum of
the closest approach
is given by
Consequently,
we can say
that the radius
of the copper nucleus
is less than
Comparing this nuclear
radius with that of
an atom,
the size of a nucleus
is less than 1/5000
of that of an atom.
You must be able
to realize
how small
the atomic nucleus is.
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[Artificial Transmutation
of Elements, Proton]
In 1919,
Rutherford succeeded
in changing nitrogen
into oxygen
artificially.
This was the first
artificial transmutation
of elements.
As part of
the alpha particle
scattering experiment,
Rutherford placed
a radioactive material
in nitrogen gas
as shown in the following
figure,
and he observed the
scintillations on the
fluorescent substance.
He found very
bright scintillations
whose directions
were not considered
to be those by alpha particles.
Similar phenomena
in various kinds
of elements,
for example,
boron, fluorine,
neon, sodium,
phosphorus, sulfur,
argon, and so on
were observed later.
These phenomena
were considered that
high speed alpha
particles collide
with atoms and knock
on an "unknown" particle
to rush out
with high energies.
Photographs of
these phenomena
were successfully
taken with
Wilson's cloud chamber.
By putting the cloud chamber
in a magnetic field
and analyzing
the results precisely,
it was clarified
that these "unknown"
particles are
the same as
the hydrogen ions.
Since a hydrogen atom
consists of an electron
and a nucleus,
a hydrogen ion
is the hydrogen atom
having lost an electron.
This is nothing else
than the
nucleus of hydrogen atom,
which was named
proton
by Rutherford.
This phenomenon
was considered
as the following process:
This means
that the nucleus
of the nitrogen atom
is disintegrated
under the strong forces
exerted
by a high speed
alpha particle,
the hydrogen nucleus
(proton) is
liberated,
and the rests
are rearranged
to form the nucleus
of oxygen.
Namely, it implies
that Rutherford
carried out
the first artificial
transmutation of element;
this is a realization
of alchemists' dream
in a modern form.
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[The Constituents of Atomic Nuclei]
Thus,
it turned out
that the proton
is one of the constituents
of atomic nuclei.
However, it was not clear
whether a nucleus
is made only of protons.
It appears quite
reasonable to consider
that the helium nucleus
(alpha particle)
consists of
four protons
and two electrons.
On the early stage,
people thought
that the nucleus
is made of protons
and electrons.
It has however been
clarified that this idea has
a serious inconsistency.
After the discovery
of the neutron
by J. Chadwick
(UK, 1891 - 1974),
it has finally
elucidated that
the true constituents
of nuclei
are protons
and neutrons.
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