Chemistry Nuclear Chemistry Chapter - Chemistry JEE Advanced

Nuclear chemistry is the study of the chemical and physical characteristics of elements that are involved in nuclear processes, or reactions that take place within the nucleus. From the research of element production in space to the design of radiopharmaceuticals for diagnostic medicine, modern nuclear chemistry, also known as radiochemistry, has become increasingly multidisciplinary in its applications. Indeed, nuclear chemists' chemical technology has grown so significant that biologists, geologists, and physicists all employ nuclear chemistry as a common tool in their fields.

Nuclear chemists work in a wide range of fields, such as nuclear imaging and nuclear technology. They frequently seek to increase the efficiency and safety of nuclear power plants, as well as the storage of radioactive materials.

Nuclear chemistry is a crucial component of the JEE exam. The themes of what is nuclear chemistry, nuclear radiation, artificially simulated nuclear reactions (fission and fusion), the uses of nuclear chemistry, and so on are all important. This article illustrates the types of questions that might be asked about this topic.

Important Topics of Nuclear Chemistry

• Types of Radiations

• Stimulated nuclear reactions

• Theory of nuclear integration

Important Definitions of Nuclear Chemistry

What is Nuclear Chemistry?

• Nuclear chemistry is the study of how changes in the structure of the atomic nucleus affect the physical and chemical characteristics of atoms. It also discusses the energy generated by nuclear reactions as well as its applications. It is also known as radiochemistry, and it encompasses the study of the production of elements in the cosmos, as well as the development of radioactive medications for diagnostic medicine and a variety of other uses.

Nuclear Reactions and Radiations

Rutherford distinguished three types of radioactive radiations in 1902 by passing them across two oppositely charged plates.

• Alpha rays are those that bend towards a negative plate and carry a positive charge.

• Beta rays are those that bend towards the positive plate and carry a negative charge.

• Gamma rays are the third type, which are uncharged and travel right through the electric field.

Types of Radiations

Nuclear reactions, unlike typical chemical reactions that produce molecules,  result in the change of one element into another. This feature of nuclear processes is employed to collect nuclear energy in nuclear power plants. The three nuclear radiations are explained below:

• Alpha Radiation

It's the part of an alpha particle's emission that comes from the nucleus of an atom. The particle is comparable to the Helium (He) nucleus in that it has two protons and two neutrons. The atomic mass of an atom reduces by 4 units when it emits a particle.

$_{92}^{238}\textrm{U}\rightarrow_{2}^{4}\textrm{He}+_{90}^{234}\textrm{Th}$

• Beta Radiation

The conversion of a neutron into a neutron and an electron is known as neutron transmutation. The mass of an atom does not change when it emits a particle. The number of atoms in the universe will rise by one.

$_{6}^{14}\textrm{C}\rightarrow _{-1}^{0}\textrm{e}+_{7}^{14}\textrm{N}$

• Gamma Radiation

The conversion of a neutron into a neutron and an electron is known as neutron transmutation. The mass of an atom does not change when it emits a particle. The number of atoms in the universe will rise by one. 

$_{27}^{60}\textrm{Co}\rightarrow_{28}^{60}\textrm{Ni}+_{-1}^{0}\textrm{e}+2_{0}^{0}\gamma{}$

Stimulated Nuclear Reactions

Nuclear processes may be purposefully promoted, unlike most elements, which decay naturally. The following are examples of these sorts of reactions.

• Nuclear Fission

A heavy nucleus divides into two lighter nuclei in a nuclear fission process that is intentionally simulated. Fission was found when a sample of Uranium-235 was bombarded with neutrons, resulting in the formation of lighter elements such as Barium. Each splitting nucleus in a typical nuclear chain reaction releases more than one neutron, which collides with nearby nuclei and initiates a series of self-sustaining nuclear fission processes. With each generation of events, the fission rate grows exponentially.

• Nuclear Fusion

Nuclear fusion is a man-made nuclear process in which two or more nuclei of different elements join to form a heavier and more stable nucleus. The fusion process demands extremely high temperatures, which can be generated by nuclear fission processes. Nuclear fusion produces enormous quantities of energy, which is used to power the sun and all other stars. Deuterium-deuterium (D-D) fusion and deuterium-tritium (D-T) fusion are two examples.

Binding Energy

• The minimal amount of energy necessary to remove a particle from a system of particles is known as binding energy. To put it another way, it's the energy that's utilised to break down a system of particles into single units. Binding energy is mostly studied in atomic Physics and Chemistry, as well as condensed matter physics. The binding energy word is used in nuclear physics to define the separation energy.

• Binding energy is required to break subatomic particles in atomic nuclei or an atom's nucleus into its constituents, neutrons, and protons, generally known as nucleons.

B.E. = ∆m × c2  erg where m is in grams and c is in cm/sec.
Also, B.E. = 931.478 ×  ∆m MeV (Mega or Million electron volt)

The Magic Numbers

• The electron shell model of atomic structure is similar to the nuclear shell model of nuclear structure. Similar to how a given number of electrons (2, 8, 18, 36, 54, and 86) leads to a stable closed-shell electron configuration, a specific number of nucleons leads to a closed shell in nuclei. Protons and neutrons have the ability to form a closed shell. Closed nuclear shells are nuclei containing 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons.

Nuclei with a closed shell are more stable than those with an open shell. Magic numbers are the numbers of nucleons that correspond to closed nuclear shells.

Theory of Nuclear Integration: Rutherford and Soddy

The ejection of α, β particles, and γ-rays from a radioactive material has been satisfactorily explained by Rutherford and Soddy.

• Step I: An excited nucleus (a nucleus of low B.E. or higher energy level) tries to attain a lower energy level in order to attain stability. Therefore, α-particles are emitted from the nucleus as an energy carrier.

• Step II: During α-decay no doubt that the energy level comes down but the n/p ratio increases; therefore, to decrease the n/p ratio and attain stability, the nucleus undergoes neutron decay or neutron transformation to show the emission of β-particles. 

• The energy of β-particles does not account for the difference in energy between the parent and daughter nuclei during neutron decay; therefore, missing energy gave the existence of another particle called anti-neutrino. 

• Step III: The resultant nucleus after α, β-emission still possesses a higher energy level than required for its stability and this difference of energy comes out in the form of γ-rays. Thus, gamma radiations are given off by the nuclei in an excited state. Therefore, α and β-emissions are primary whereas γ-emissions are secondary emissions.

• The Rutherford-Soddy law tells us about the radiation and decay of elements.

1) The radioactivity is based on the counting of disintegration per second. The activity depends on the number of decays per second.

2) The law is the exponential decay law which states that a fixed fraction of elements will decay per unit of time.

N=N0e−λt

where N = size of population of radioactive element , N₀ = size of initial population of radioactive element, 𝝀 = decay constant.

This shows that the population decays exponentially at a rate that depends on the decay constant.

The time required for half of the population to decay is called the half-life which is given by                              

t1/2 = 0.693/λ

Radioactive Decay

• Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. Alpha, beta, and gamma decays are the three most common types of decays, these decays involve emitting one or more particles or photons.

• Radioactive disintegration is an example of first-order reaction, i.e., the rate of decay is directly proportional to the no. of atoms (amount) of the element present at that particular time.

A → Decay product        (N0 = No. of atoms at t = 0)

                                           (N = No. of atoms left after t = t) 

–dN/dt ∝ [N]1= λN. The negative sign indicates the decreasing trend of N with increasing time.

Where λ is the proportionality constant and is known as decay or disintegration or radioactive constant. 

Integration of this equation gives,

–ln N = λ + A (A is integration constant)

At t = 0, N = N0 

∴ A = –ln N0 

∴ –ln N = λ – ln N0

λ = (2.303/t ) log10( N0 /N) 

Solved Examples from the Chapter

Example 1: Half-life for a radioactive substance is 5 hours. Calculate the % left in 2.5 and 10 hours? 

(a)  25%

(b)  55%

(c)  40%

(d)  75%

Solution: 

T = n × t1/2

where n = (2.5/5) 

∴ Amount left in 2.5 hours = 100/(2)1/2

Also, T = n × t1/2     

where n = (10/5) 

∴ Amount left = 100/(2)2  = 25%

Hence option (a) is correct. 

Key point to remember: T = n × t1/2  

Example 2: When a radioactive substance is subjected to a vacuum, the rate of disintegration per second 

(a) Increases only if the products are gaseous

(b) Increases considerably

(c) Decreases

(d) Is not affected

Solution: The rate of disintegration is dependent on the number of radioactive atoms present in the element only.

So, in a vacuum, the rate of disintegration is not affected.

Hence the correct option is (d).

Key point to remember: The radioactive disintegration rate is given by:

dN/dt = λN     where N = no. of undecayed atoms at time t.

Solved Examples from Previous Year Questions

Question 1: Calculate the binding energy per nucleon for an alpha particle whose mass is 4.0028 amu, mp = 1.0073, and mn = 1.0087. 

(a) 6.80 MeV

(b) 7.40 MeV

(c) 5.80 MeV

(d) 10.80 MeV

Solution: Mass defect can be calculated by calculating the mass difference between a nucleus and its constituent nucleons. From the calculated mass defect, binding energy can be calculated as B.E = 931.478 × ∆m’ MeV

Mass of an α-particle = Mass of 2p + Mass of 2n = 2 × 1.0073 + 2 × 1.0087 = 4.032 amu

Actual mass of α-particle = 4.0028 amu

Mass decay = 4.032 – 4.0028 = 0.0292 amu.

Also, B.E. = 0.0292 × 931.478 = 27.20 MeV

B.E./nucleons = (27.19/4) = 6.80 MeV

Hence, the correct option is (a).

Trick: B.E = 931.478 × ∆m’ MeV

Question 2: Which of the following processes given below gives rise to 

(i) an increase in atomic no. (ii) an increase in n/p ratio (iii) a decrease in atomic no. (iv) a decrease in n/p ratio (v) emission of X-rays definitely.

(a) α-emission 

(b) β-emission 

(c) positron emission 

(d) K-electron capture

Solution: 

(i) Increase in atomic number: β-emission

(ii) Increase in n/p ratio: α-emission, K-electron capture

(iii) Decrease in atomic number: positron emission, α-emission

(iv) Decrease in n/p ratio: β-emission

(v)  X-rays emission: K-electron capture

Trick: To be familiar with the various nuclear radiations.

Question 3: Isotope(s) of hydrogen which emits low energy β- particles with t1/2 value > 12 years is/are

(a) Isobaric

(b) Isochoric

(c) Isothermal

(d) Adiabatic

Solution: 

Tritium (31H or T) is an isotope of hydrogen which is radioactive and emits low energy β-particles. 

It has one proton and two neutrons in the nucleus while 11H and 21H are stable while 31H is radioactive. 

Hence, option (b) is correct.

Trick: Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation.

Practice Questions

Question 1: Increase in atomic number is observed during

A) Alpha emission

B) Beta emission

C) Positron emission           

D) Electron capture

Answer: Option (B) Beta emission

Question 2: α-rays have     

A) Positive charge

B) Negative charge

C) No charge

D) Sometimes positive charge and sometimes negative charge

Answer: (a) Positive charge

Conclusion

Nuclear chemistry is a branch of chemistry that studies changes in the nucleus of atoms of different elements. These nuclear alterations are a source of nuclear energy and radioactivity, and the energy supplied by nuclear reactions has a wide range of uses.

A range of concepts are involved, and calculations based on these concepts aid in analysing changes in the environment, surroundings, or experiment, as well as a deeper knowledge of a phenomenon. As a result, this is crucial not only for competitive tests like JEE or NEET but also for a deeper grasp of nature.