We know that matter is made up of atoms. We also know that in
atom, at the Center, mass is concentrated with the combination of protons and
neutrons. That central part of the atom is called nucleus and the elements of
the nucleus are called nucleons. We also know that electrons revolve around the
nucleus in specified orbits.
The number of the protons inside the nucleus is called atomic
number. The total number of the nucleons inside the nucleus is equal to the sum
of number of the protons and the number of the neutrons. This total number of
the nucleons is called mass number. The number of the nucleons present inside
the nucleus is the difference between the mass number and the atomic number.
The conversion of one element into the other element by artificial means is
called artificial transmutation of element.
Discovering neutrons has taken much longer time than the
discovery of electrons and protons. It is simply because they are not having a
charge and hence identifying their presence has taken a little bit longer time.
A scientist by the name James Chadwick has discovered the neutrons.
When a helium particle is allowed to strike a beryllium atom,
carbon atom is farmed which is in unstable state. It further splits into a
stable carbon atom and a neutron. The problem with the identification of the
neutron is it is not having charge. Therefore it is allowed to pass through
paraffin material which is a rich resource of protons.
Whenever a neutron
strikes the paraffin, one proton is released. Identifying the proton is very
easy with electronic counter. The number of the neutrons released in this
process is nothing but equal to the number of the protons. The experimental
arrangement is as shown in the block diagram.
Properties of neutrons
- It has no charge.
- It is not influenced by electric and magnetic fields as it is not having a charge.
- It has a high penetration power so that it can penetrate deep into the materials.
- It has low ionization ability.
- It is stable inside the nucleus but not stable outside the nucleus. It has average life of thousand seconds outside the nucleus. It means once if it is outside the nucleus it can stay like the neutron only for thousands seconds and within that time it converts into anti-neutrino, electron and proton.
- The speed of the neutron can be slowed down by passing it through heavy water.
- The mass of the neutron is approximately equal to mass of the proton.
Classification of nuclei
The nuclei having the same atomic number but different mass
number are called isotopes. They have same number of protons but different
number of neutrons.
The nuclei having the same mass number but different atomic
number are called isobars. Here total number of the nucleons is same but the
number of the protons and neutrons are different in each case.
The nuclei having the same number of the neutrons are called
isotones. They have different mass numbers and different atomic numbers but the
difference between mass number and atomic number is same.
In the following attracted paper the examples of this
different kinds of nuclei are given.
To measure the mass of fundamental particles like electrons
and protons the regular systems like standard international system are not
directly useful. As these masses are very small, in accordance to them, small
unit of mass is defined and that unit is called atomic mass unit. One atomic
mass unit is defined as one by 12th of the mass of the carbon atom.
Nuclear size
Nucleus consists of both protons and neutrons. Neutrons are
neutral particles whereas the protons are having positive charge. The total
charge of the nucleus is equal to the product of number of the protons and the
charge of each proton. In terms of magnitude charge of proton and electron are
equal. But the proton has a positive charge whereas the electron has a negative
charge.
There is a lot of empty space inside the atom. The size of
the nucleus is very small when compared with the size of the atom. The size of
the nucleus is directly proportional to number of nucleons. It is simply
because the nucleus is nothing but the group of the nucleons.
Therefore we can
conclude that the volume of the nucleus is directly proportional to mass
number. We can also assume that the nucleus is approximately spherical in
shape. Basing on this we can derive the relation between the radius of the
nucleus and the mass number as shown below. We can also express this radius in
terms of the mass number as shown below.
Application
A stationary nucleus splits into two lighter nuclei. Find the
ratio of velocity of the two elements of the nuclei?
As there is no external force acting on the system, the
linear momentum of the system is always conserved. Initially the nucleus is
being in the state of rest and hence its initial momentum equal to 0. After the explosion both the
particles will have same the momentum in the opposite direction. Hence we can
express the equation for velocity and kinetic energies as shown below.
We have already explained that the mass of the fundamental
particles can be measured with the unit called atomic mass unit. Similarly to
measure their corresponding energies we need to use a unit called electron
volts. We know according to Einstein’s mass energy relation that each mass will
have correspondingly some energy.
In the above diagram the conversion is made and it is proved
that one atomic mass unit is having energy equal to 931.5 million electron volt.
Nuclear density
We know that density is defined as mass per unit volume.
Density of the nucleus is also defined as the ratio of total mass of the
nucleus to its volume. By substituting the expression for the radius in the
above equation and by simplifying we can calculate that the density of the
nucleus. It is a very high value as shown below. It is also proved that the
density of the nucleus is independent of mass number. It means whatever may be
the name of the nuclei; the density is going to be the same. The derivation is
shown below.
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