Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur

B.Sc. I year (Sem-II)Unit III (Physical Chemistry)

Topic- (A) Nuclear Chemistry

By

Ms. Swati A. Tandekar

Tai Golwalkar Mahavidyalaya , RamtekDepartment of Chemistry

Contents (According to syllabus) :

• Composition of Nucleus

• Mass defects

•Nuclear binding energy

•Average binding energy per nucleon

•Explanation of nuclear stability on the basis of graphbetween average binding energy per nucleon andatomic mass number

• Nuclear reactions: Fission and Fusion

•Nuclear models: Shell model and Liquid drop model

•Bohr- Wheelar theory

•Application of Radioisotopes

Nuclear chemistry is the study of reactions that involve

changes in nuclear structure.

Composition of NucleusComposition. The nucleus of an atom is made up of protons andneutrons (two types of baryons) joined by the nuclear force. Thesebaryons are further made up of sub-atomic fundamental particlesknown as quarks joined by the strong interaction.

Nuclides are represented in chemical notation by a subscript atomic number (Z) and superscript nucleon number (A) on the left side of the element’s symbol (X):

Nuclei can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped.

Mass defectsThe mass defect of a nucleus represents the mass of theenergy binding the nucleus, and is the difference between themass of a nucleus and the sum of the masses of the nucleonsof which it is composed.

Since energy and mass are related based on the following equation-

E = mc2

Where c is the speed of light. In nuclei, the binding energy is so highthat it holds the considerable amount of mass.

The actual mass is less than the sum of individual masses of theconstituent neutrons and protons in every situation because energyis ejected when the nucleus is created. This 'missing mass' is knownas the mass defect.

Mass defect is determined as the difference between the atomicmass observed and expected by the combined masses of its protonsand neutrons

Nuclear Binding energy

•Nuclear binding energy is the minimum energy thatwould be required to split the nucleus of an atom into itscomponent parts. These component partsare neutrons and protons, which are collectivelycalled nucleons.

•The binding energy is always a positive number. since allnuclei require net energy to separate them into individualprotons and neutrons.

•Nuclear binding energy is the energy required to split anucleus of an atom into its components.

•Nuclear binding energy is used to determine whetherfission or fusion will be a favorable process.

Average binding energy per nucleon

The binding energy per nucleon of a nucleus is the bindingenergy divided by the total number of nucleons.

Measure of stability of the nucleus. Larger the bindingenergy per nucleon, the greater the work that must be doneto remove the nucleon from the nucleus, the more stablethe nucleus

The relative stability of a nucleus is correlated with its bindingenergy per nucleon, the total binding energy for the nucleus dividedby the number or nucleons in the nucleus.For instance, we saw in Example 2 that the binding energy fora helium nucleus is 28.4 MeV. The binding energy per nucleon fora helium nucleus is therefore:28.4MeV 4nucleons=7.10MeV/nucleon

Explanation of nuclear stability on the basis of graph between

average binding energy per nucleon and atomic mass number Nuclear binding energy is also used to determine whether fission or fusion willbe a favorable process. For elements lighter than iron-56, fusion will releaseenergy because the nuclear binding energy increases with increasing mass.Elements heavier than iron-56 will generally release energy upon fission, as thelighter elements produced contain greater nuclear binding energy. As such, thereis a peak at iron-56 on the nuclear binding energy curve.

This graph shows the nuclear binding energy (in MeV) per nucleon as a functionof the number of nucleons in the nucleus. Notice that iron-56 has the mostbinding energy per nucleon, making it the most stable nucleus.

Nuclear reactions: Fission and Fusion(A) Nuclear Fission

Nuclear fission is a nuclear reaction or a radioactive

decay process in which the nucleus of an atom splits into two

or more smaller, lighter nuclei. The fission process often

produces gamma photons, and releases a very large amount

of energy even by the energetic standards of radioactive decay.

(B) Nuclear FusionNuclear fusion is the process of making a single heavy nucleus (part ofan atom) from two lighter nuclei. This process is called a nuclearreaction. It releases a large amount of energy.

The nucleus made by fusion is heavier than either of the starting nuclei.However, it is not as heavy as the combination of the original mass of thestarting nuclei (atoms).

Fusion happens in the middle of stars, like the Sun. Hydrogen atoms arefused together to make helium. This releases lots of energy. This energypowers the heat and light of the star. Not all elements can be joined.Heavier elements are less easily joined than lighter ones

Nuclear models: Shell model and Liquid drop model

(A) Shell modelThe nuclear shell model is a model of the atomic nucleus. It uses the Pauliexclusion principle to explain the nucleus structure in terms of energy levels.

It basically explains the distribution of energy level into different atom shellsand nucleus atom shells. A shell is described as the energy level whereparticles having the same energy exists.

In this model, all the nuclear particles are paired one-to-one, neutron withneutron and proton with a proton. The paired neutrons and protons innuclear energy levels are filled when the number of neutrons or protons isequal to 2, 8,20,28,50,82 or 126. These are the magic numbers which showthe most stable nuclei.

The unpaired ones are responsible for the properties of a nucleus andvalence electrons are responsible for different chemical properties ofelements.

With the help of the shell model, we can accurately predict the properties ofnuclei such as angular momentum.

But, for nuclei which are in a highly unstable state, the shell model needs tobe modified or replaced with other models such as the collective model,liquid-drop model and compound nucleus model.

(B) Liquid drop modelAccording to this model, the atomic nucleus behaves like themolecules in a drop of liquid. But in this nuclear scale, thefluid is made of nucleons (protons and neutrons), which are heldtogether by the strong nuclear force.

The liquid drop model of the nucleus takes into account the factthat the nuclear forces on the nucleons on the surface aredifferent from those on nucleons in the interior of the nucleus.

The interior nucleons are completely surrounded byother attracting nucleons. Here is the analogy with the forcesthat form a drop of liquid.

In the ground state the nucleus is spherical. If the sufficientkinetic or binding energy is added, this spherical nucleus may bedistorted into a dumbbell shape and then may be splittedinto two fragments.

Since these fragments are a more stable configuration, thesplitting of such heavy nuclei must be accompanied by energyrelease. This model does not explain all the properties of theatomic nucleus, but does explain the predicted nuclear bindingenergies.

Born-Wheelar TheoryIn essence, the Bohr-Wheeler theory is simple. Two forces are at work in a heavynucleus: the nuclear force, holding the nucleus together, and the Coulombelectrical force, tending to blow the nucleus apart (see the figure below). For allthe nuclei we know, the nuclear force is in control, but for the heaviest knownnuclei, it is only barely in control. The problem of the fissionability of a nucleus canbe posed this way: If a nucleus is stretched into an elongated shape, what isgreater—the repulsive electric force tending to push it into an even moreelongated shape, or the attractive nuclear force tending to restore it to a sphericalor near spherical shape? If a nucleus like uranium is slightly distorted from itsnormal shape, the nuclear force wins out, tending to restore it to its originalshape. If it is distorted much further, the electric force wins out and it splits in two(or occasionally three). Between these two regions is an energy barrier. In slowspontaneous fission (a rarity in naturally occurring elements), this barrier can bepenetrated, just as a barrier is penetrated in alpha decay. For the rapid fission thatoccurs in reactors or bombs, the barrier must be surmounted. The magnitude ofthe energy barrier to be overcome depends sensitively on the relative magnitudeof two energies of opposite sign: the Coulomb (electric) energy, arising from themutual repulsion of the protons; and the surface-tension energy, arising from thenuclear forces. From approximate considerations of these energies, one canextract a significant parameter, which measures nuclear fissionability.

Application of Radioisotopes

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