Nuclear forces and Energy levels

Introduction:

Two types of particles are present in the nucleus of every atom, neutrons and protons called nucleons.
The force that act between these nucleons is knowns as nuclear force, also knowns as strong forces or
interaction forces. It is always attractive and acts between proton-proton, neutron-neutron and
neutron-proton. These forces keep the protons and neutrons binding with each other and keep the
nucleus of an atom in stable position.

The nuclear force is very much stronger (about 10 million times) than the chemical forces exist between
atoms or molecules and this is the reason that the energy released during nuclear reactions is million
times more than the energy produced in the chemical reactions.

These forces are very short range forces i.e. only a few femtometer (10^-15m) after which they
decreases abruptly.

Nuclear Models:

There are three models which explains the structure of nucleus and thus these nuclear forces.

1. The liquid drop model:

According to this nuclear model, the nucleus of an atoms acts the same as molecules in a liquid drop.
The liquid drop in this case is made up of protons and neutrons which are bind together by nuclear
forces.

Also according to this model, the nuclear forces acting on nucleons are different on those which exist at
the surfaces while different on those which exist in the interior of the nucleus. The reason is that the
nucleons in the center are surrounded by other protons and neutrons also which is analogy to the same
phenomenon which exist in liquids.

Assumptions of this model:

• The nucleus volume is directly proportional to the total number of its nucleons

• The density of the nucleus is almost constant

• The binding energy of the nucleons increases as the number of nucleons increases

This model is unable to explain all the characteristics of the nucleus of an atom but still is able to explain
the nuclear binding energies.

2. Shell model:

It is the most simple model which explains the arrangement of nucleons i.e. protons and neutrons, in
separate systems of shells similar to the atomic model which shows arrangement of electrons around
the nucleus of an atom.

1
Assumptions of this model:

• Protons and neutrons are present in separate shells in both the light and heavy atoms.

• Each shell is occupied with a pair of neutrons or protons having opposite spin.

• Motion nucleon depends on the average attractive force of the other nucleons surrounding it.

This model is very helpful in demonstrating the relationship between the number of nucleons and the
stability of the nucleus.

3. The collective model

It is also known as unified model. This model incorporates both the aspects of liquid drop model and the
shell model in order to describe those electric and magnetic properties of the nucleus which weren’t
explained by the two models separately.

This model consider both the motion of protons and neutrons i.e. outside the closed shells as well as the
motion of protons and neutrons inside the core.

Strong Vs Weak nuclear forces:

Strong nuclear forces are those forces which hold the nucleons together whereas weak nuclear forces
are those forces which result in radioactive decay of an atom.

The strong nuclear force is a short range force which binds the protons and neutrons in the nucleus of
an atom. Its good example is fusion process.

Weak nuclear force is also a short range force which can convert a neutron into a proton resulting in the
emission of light particles. Its good example is the process of nuclear decay.

Nuclear Mass defect:

It is the difference between the overall mass of the nucleus and the sum of the masses of protons and
neutrons.

It can be calculated by determining the difference between the atomic mass of an atom and the sum of
the mass of its protons and neutrons. This difference in the mass is converted into nuclear binding
energy which can be calculated by Einstein’s equation i.e. E=mc2

The actual mass of the nucleus of an atom is always less than the combine mass of its nucleons. The
reason is that energy is emitted when protons and neutrons combine to form a nucleus.

Nuclear disintegration:

The process in which a nucleus is bombarded with particles like alpha, neutrons, protons etc. making the
nucleus unstable, which then emits a proton or neutron or any other particle along with some energy
making the nucleus stable again, is knowns as nuclear disintegration.

Fission reaction:

2
The process in which a heavy atomic nucleus is split into two light particles of almost equal masses,
along with the release of huge amount of energy.

This process which is sometimes spontaneous or sometimes required some kind of excitation, results in
the formation of radioactive products, colossal amount of energy is released and a few neutrons are also
released. These released neutrons can further cause fission of other nearby fissionable particles which
results in more emission of neutrons and energy. The process carries on and result in a chain reaction in
which a huge number of nuclei experience fission reaction and enormous amount of energy is released.

This reaction can also be used for peaceful purposes like electricity and isotopes production in nuclear
reactor and for destructive purposes like atomic bomb.

Fusion reaction:

It is a nuclear reaction in which light nuclei combine and form heavier and stable nuclei along with the
emission of a huge amount of energy.

It is the opposite process of nuclear fission. Sun and other stars produce light and heat due to this
process. This reaction occurs mostly when the lightest element hydrogen and its isotopes combine
together.

For this reaction to occur, the two nuclei must be brought so nearby that they activate the nuclear
forces and glue them together. This reaction also requires an environment of high density and high
temperature.

This reaction can also be used for peaceful purposes like energy production and for destructive purposes
like hydrogen bomb.

Nuclear Energy Level:

Quantum mechanical laws are used in order to study the characteristics of nucleus. These laws compel
the nucleus to be present in a finite number of states, defined on the basis of the energy possessed by
these states, and are known as nuclear energy levels.

When a nucleus is in isolation, its energy is minimum and the state is called ground state. Similarly when
a nucleus is in a different state, it has extra amount of energy which is released in the form of high
energy alpha or gamma photon which compels the nucleus to shift into ground state once again.

The shell model can be used to describe these energy levels. It describes how much energy is required to
move nucleons from one orbit to another which then changes the quantum number as every state has
its unique quantum number.

In order to calculate the transition rates among different energy levels and half-life of the decay,
specialized alpha, gamma and beta detectors along with associated electronic circuitry is required.

Radioactivity:

3
The process in which an unstable nuclei emits certain type of energy particles in the form of radiation is
called radioactivity. It is also known as nuclear or radioactive decay. The most commonly known
radiations are alpha, beta and gamma rays.

It occurs in some naturally existing elements as well as can be produced artificially. This phenomenon is
measured in terms of half-life of that element. It is the time in which the nuclei decayed to its half. The
radioactive element is called the parent element while the isotope obtained after decaying process is
called the daughter isotope.

Isotope means those elements which have same atomic number but different atomic masses. They have
same chemical properties but are different in their physical properties. Every element has one or more
isotopes.

Conclusion:

The whole topic/course is to study the nucleus of an atom, its nucleons and their arrangements in
various energy levels or states by providing evidences from different nuclear models, different types of
nuclear reactions and forces holding these nucleons together and the process of radioactivity.

A nucleus is consist of nucleons which are bind together by strong and short range nuclear forces. These
nucleons reside in different energy states or shells as predicted by different nuclear models like shell
model or liquid drop model. Nucleus with filled shells are stable while those with extra nucleons are in
excited state which then release certain particles along with energy and becomes stable. A stable
nucleus can also be made unstable when hit by nuclear particles which result in either radioactivity or
nucleus disintegration or fission reaction. Also the total mass of a nucleus is less than the sum of masses
of its nucleons, result in mass defect. This mass defect accounts for the nuclear binding energy. Huge
amount of energy is released during both the fission and fusion reactions which can be used for both the
peaceful and destructive purposes.

The research related to the nucleus structure, its nucleons arrangements and reactions is a very wide
and unlimited field. Exploration in this field is unending and new facts and hypothesis originates with the
ongoing experiments throughout the world.

References
 Business, Technology. (2007, September 14). Nuclear Forces. Retrieved from SlideShare:
https://www.slideshare.net/furrs/nuclear-forces-pp

 Centre for Nuclear science and Technology Information. (2018). Nuclear fusion. Retrieved from
Know Nuclear: http://www.nuclearconnect.org

 Contemporary Physics Education Project CPEP. (n.d.). A guide to the Nuclear Wall Chart. 2004.
Retrieved from http://www2.lbl.gov/abc/wallchart/guide.html

 Ellis P.Steinberg. (2020). Matter and Energy. Retrieved from Britannica:

4
 Ruprecht Machleidt. (2013). Nuclear Forces. Retrieved from Scholarpedia:
http://www.scholarpedia.org