Unit 21

Applied Chemistry

Class 12 Chemistry

Nuclear Chemistry and Applications of Radioactivity

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NEB/CDC syllabus scope: Unit 21 is a 2-teaching-hour Applied Chemistry chapter. It includes natural and artificial radioactivity; units of radioactivity; nuclear reactions; nuclear fission and fusion; nuclear power and nuclear weapons at a descriptive level; industrial uses of radioactivity; medical uses of radioactivity; radiocarbon dating; and harmful effects of nuclear radiation.

1. Introduction to Nuclear Chemistry

Nuclear Chemistry Nuclear chemistry is the branch of chemistry concerned with changes in atomic nuclei, radioactivity, nuclear reactions, radioisotopes and their applications.

Ordinary chemical reactions mainly involve rearrangement of electrons and chemical bonds. In nuclear reactions, the nucleus itself changes, so one isotope or even one element may be converted into another.

Feature	Chemical reaction	Nuclear reaction
Main particles involved	Valence electrons	Nucleus: protons/neutrons
Element identity	Usually unchanged	May change
Energy change	Relatively small	Can be extremely large
Effect of temperature/pressure	Often important	Radioactive decay rate is largely independent of ordinary chemical conditions
Mass change	Usually negligible	Small mass differences can correspond to large energy release

Diagram 1: Chemical reactions vs nuclear reactions

2. Radioactivity

Radioactivity Radioactivity is the spontaneous transformation of an unstable atomic nucleus accompanied by emission of ionizing radiation such as alpha particles, beta particles and/or gamma rays.

Unstable nucleus → more stable nucleus + radiation

Radioactive decay is a nuclear phenomenon. Its rate is generally not changed by ordinary variations in temperature, pressure or chemical state.

3. Natural and Artificial Radioactivity

3.1 Natural Radioactivity

Natural radioactivity occurs spontaneously in naturally occurring unstable isotopes.

Example: Alpha decay of uranium-238

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

3.2 Artificial Radioactivity

Artificial radioactivity is produced when a stable or long-lived nucleus is transformed into a radioactive nucleus by bombardment with suitable particles.

Classic example

²⁷₁₃Al + ⁴₂He → ³⁰₁₅P + ¹₀n

The phosphorus-30 produced is radioactive.

Feature	Natural radioactivity	Artificial radioactivity
Origin	Occurs naturally	Radioisotope produced artificially
Initial nucleus	Naturally unstable nucleus	Nucleus made radioactive by nuclear reaction
Example	Uranium/radium decay	³⁰P produced from ²⁷Al bombardment
Basic phenomenon	Spontaneous decay	Induced production followed by radioactive decay

Diagram 2: Natural and artificial radioactivity

4. Alpha, Beta and Gamma Radiations

Radiation	Nature	Charge	Relative ionizing power	Relative penetration
Alpha, α	Helium nucleus, ⁴₂He²⁺	+2	High	Low
Beta, β⁻	High-speed electron	−1	Moderate	Moderate
Gamma, γ	High-energy electromagnetic photon	0	Lower per traversal than α	High

4.1 Alpha Decay

Mass number decreases by 4 and atomic number decreases by 2.

ᴬZX → ᴬ⁻⁴Z₋₂Y + ⁴₂He

4.2 Beta-Minus Decay

A neutron in the nucleus changes into a proton, emitting an electron and an antineutrino. Mass number is unchanged; atomic number increases by 1.

¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄

4.3 Gamma Emission

An excited nucleus loses excess energy by emitting a gamma photon. Neither mass number nor atomic number changes.

Diagram 3: General penetrating-power comparison

5. Units of Radioactivity

Activity Activity is the number of nuclear disintegrations occurring per unit time in a radioactive sample.

Unit	Symbol	Definition / relation
Becquerel	Bq	1 Bq = 1 disintegration per second = 1 s⁻¹
Curie	Ci	1 Ci = 3.7 × 10¹⁰ Bq
Rutherford	Rd	1 Rd = 10⁶ Bq

SI unit The SI unit of radioactive activity is the becquerel (Bq).

Worked Example

A sample undergoes 5.0 × 10⁶ disintegrations per second.

Activity = 5.0 × 10⁶ Bq = 5.0 Rd

6. Nuclear Reactions

Nuclear reaction A nuclear reaction is a process in which an atomic nucleus is transformed, often through interaction with another nucleus or particle.

6.1 Conservation Rules

For a correctly balanced nuclear equation:

Total mass number (A) is conserved.
Total atomic number / nuclear charge (Z) is conserved.
Energy, momentum and other relevant physical quantities are conserved.

Balancing Example ²⁷₁₃Al + ⁴₂He → ³⁰₁₅P + ¹₀n

Mass numbers: 27 + 4 = 30 + 1 = 31.

Atomic numbers: 13 + 2 = 15 + 0 = 15.

Diagram 4: Conservation of A and Z in nuclear equations

7. Nuclear Fission

Nuclear Fission Nuclear fission is the splitting of a heavy nucleus into two medium-mass nuclei, usually with emission of neutrons and a large release of energy.

Representative example

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + energy

The emitted neutrons can induce further fissions. If this process continues, it forms a chain reaction.

Diagram 5: General idea of a fission chain reaction

8. Nuclear Fusion

Nuclear Fusion Nuclear fusion is the joining of light nuclei to form a heavier nucleus, accompanied by release of a large amount of energy.

Deuterium–tritium example

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

Fusion requires extremely high temperature and suitable confinement because positively charged nuclei repel one another. Fusion reactions power the Sun and stars.

Diagram 6: General deuterium–tritium fusion reaction

9. Difference Between Nuclear Fission and Fusion

Feature	Fission	Fusion
Basic process	Heavy nucleus splits	Light nuclei combine
Typical fuel example	U-235	Hydrogen isotopes
Condition	Can be initiated by neutron absorption	Requires extremely high temperature/confinement
Chain reaction	Possible through emitted neutrons	Not the same neutron-chain mechanism as fission
Energy source	Mass defect / increased binding energy	Mass defect / increased binding energy
Natural example	Some spontaneous fission occurs in heavy nuclei	Sun and stars
Power technology	Used in present commercial nuclear reactors	Controlled power generation remains technologically difficult

10. Nuclear Power

Nuclear power plants use a controlled fission chain reaction to produce heat. The heat is transferred to a working fluid/steam cycle that drives a turbine connected to an electrical generator.

10.1 Main Functional Parts of a Fission Reactor

Fuel: fissile material in suitable fuel assemblies.
Moderator: in many reactor designs, slows neutrons to improve probability of further fission.
Control rods: absorb neutrons and regulate the chain reaction.
Coolant: carries heat away from the core.
Steam/turbine system: converts thermal energy into mechanical and electrical energy.
Containment and shielding: reduce release/exposure hazards.

Diagram 7: High-level nuclear power generation process

10.2 Advantages

Very large energy release from relatively small fuel mass.
Low direct carbon-dioxide emissions during reactor operation.
Can provide continuous large-scale electricity when operated safely and reliably.

10.3 Limitations and Risks

Radioactive waste requires secure long-term management.
Accidents, although uncommon, can have serious consequences.
Plants require stringent safety, security and regulatory systems.
Construction, decommissioning and waste-management costs can be high.

11. Nuclear Weapons — Descriptive Overview

Nuclear weapons release nuclear energy extremely rapidly rather than controlling it for steady power production. Broadly, nuclear weapons may involve fission, or combinations of fission and fusion.

Safety and scope This chapter only requires a descriptive distinction between nuclear power and nuclear weapons. Detailed weapon design, construction or operational information is neither necessary for NEB study nor included here.

Nuclear power	Nuclear weapon
Controlled energy release	Extremely rapid, destructive energy release
Designed for electricity/heat production	Designed as a weapon
Requires continuous control, cooling and containment	Not a controlled power-generation system
Peaceful energy application	Severe blast, heat and radiation consequences

12. Industrial Uses of Radioactivity

Use	How radioactivity helps
Industrial radiography	Penetrating radiation reveals internal flaws in welds and metal components
Thickness gauging	Radiation transmission changes with sheet thickness
Level measurement	Radiation attenuation indicates liquid/material level in closed vessels
Tracer studies	Radioisotopes track flow, mixing, leakage or wear
Process control	Radiation detectors can monitor density or composition in suitable systems
Sterilization / irradiation	Ionizing radiation can reduce microorganisms in selected industrial products

Diagram 8: Common industrial applications

13. Medical Uses of Radioactivity

Radioisotopes are used in medicine for both diagnosis and treatment. Selection depends on half-life, radiation type, biological behavior and clinical purpose.

Application	Typical isotope/example	Purpose
Diagnostic imaging	Technetium-99m	Tracer imaging of organs/tissues
Thyroid diagnosis/treatment	Iodine radioisotopes such as I-123/I-131 depending on purpose	Thyroid uptake/imaging or therapy
Radiotherapy	Selected radioisotopes including Co-60 in appropriate systems	Deliver ionizing radiation to destroy malignant cells
Sterilization	Ionizing radiation	Sterilization of selected medical products

Medical safety Radioisotopes used in medicine require specialist prescription, dosimetry, shielding and radiation-safety procedures. The examples above are academic classifications, not treatment advice.

14. Radiocarbon Dating

Radiocarbon dating Radiocarbon dating estimates the age of once-living organic material by measuring the remaining amount or activity of radioactive carbon-14 relative to an appropriate reference.

14.1 Formation of Carbon-14

Cosmic-ray-produced neutrons interact with atmospheric nitrogen:

¹⁴₇N + ¹₀n → ¹⁴₆C + ¹₁H

Carbon-14 becomes part of atmospheric carbon dioxide and enters living organisms through the carbon cycle.

14.2 After Death

While alive, an organism exchanges carbon with the environment. After death, carbon exchange stops and the C-14 already present decays gradually.

¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄

The half-life of C-14 is about 5730 years. The lower the remaining C-14 fraction, the older the sample, within the practical dating range and subject to calibration/measurement limits.

Diagram 9: Principle of radiocarbon dating

15. Half-Life Concept

Half-life Half-life is the time required for the number of undecayed radioactive nuclei, or the activity of a radioactive sample, to fall to one-half of its initial value.

N / N₀ = (1/2)ⁿ

where n is the number of elapsed half-lives.

Elapsed half-lives	Fraction remaining	Percentage remaining
0	1	100%
1	1/2	50%
2	1/4	25%
3	1/8	12.5%
4	1/16	6.25%

Radiocarbon Example

If a sample contains 25% of the C-14 activity expected for a comparable modern sample, 2 half-lives have elapsed.

Age ≈ 2 × 5730 = 11,460 years

This is the idealized textbook calculation; real radiocarbon dating uses calibration and careful sample preparation.

16. Harmful Effects of Nuclear Radiation

Ionizing radiation can remove electrons from atoms and molecules. In living tissue, this can damage important biomolecules including DNA.

Type of effect	Examples
Cell and tissue injury	Damage to rapidly dividing cells, skin and internal tissues at sufficient doses
DNA damage	Mutations or chromosome damage
Cancer risk	Risk increases with cumulative ionizing-radiation exposure
Acute radiation syndrome	Can occur after very high whole-body exposure
Reproductive/developmental effects	Possible damage to reproductive cells or developing tissues depending on dose and timing
Environmental contamination	Long-lived radionuclides can contaminate soil, water or food chains if released

Diagram 10: General pathway from ionizing radiation to biological harm

17. Basic Radiation Protection

Radiation protection is commonly summarized using three principles:

Time

Reduce unnecessary time near a radiation source.

Distance

Increase distance from the source whenever practical.

Shielding

Use suitable shielding material for the radiation involved.

Control & monitoring

Use trained procedures, dosimetry, secure sources and regulatory controls.

Important Radioactive sources should only be handled in properly authorized facilities by trained personnel using appropriate radiation-protection procedures.

18. High-Yield Summary Table

Topic	Key idea	Exam memory point
Natural radioactivity	Spontaneous decay of naturally unstable nuclei	U, Ra and related isotopes
Artificial radioactivity	Radioisotope produced by nuclear bombardment	Induced production
Becquerel	Activity unit	1 Bq = 1 s⁻¹
Curie	Older activity unit	1 Ci = 3.7 × 10¹⁰ Bq
Fission	Heavy nucleus splits	Can give chain reaction
Fusion	Light nuclei combine	Requires extreme temperature
Nuclear power	Controlled fission heat → electricity	Control + cooling + containment
Industrial uses	Gauging, tracers, radiography	Measurement / inspection
Medical uses	Imaging and therapy	Radioisotope choice depends on purpose
Radiocarbon dating	Measure remaining C-14	Half-life ≈ 5730 y
Radiation harm	Ionization can damage cells/DNA	Time, distance, shielding

19. Common Exam Mistakes

Writing radioactivity as an electronic phenomenon. It is a nuclear phenomenon.
Confusing natural radioactivity with artificial radioactivity.
Forgetting that alpha emission lowers mass number by 4 and atomic number by 2.
Writing that beta-minus emission changes mass number. The mass number remains unchanged.
Writing that gamma emission changes atomic number. It does not.
Confusing ionizing power with penetrating power. In general, α ionizes strongly but penetrates weakly.
Writing 1 Bq as one decay per minute. It is one disintegration per second.
Writing an incorrect Curie conversion. 1 Ci = 3.7 × 10¹⁰ Bq.
Failing to balance both mass number and atomic number in nuclear equations.
Confusing fission and fusion.
Writing fusion as the process currently used in ordinary commercial nuclear fission reactors.
Describing nuclear power as an uncontrolled chain reaction. It depends on controlled fission.
Giving construction details for nuclear weapons; the syllabus only requires descriptive understanding.
Confusing HCN/chemical cyanide topics with nuclear radioactivity—these are unrelated concepts.
Forgetting that C-14 dating applies to once-living carbon-containing material, not every type of rock.
Using the C-14 half-life incorrectly; after two half-lives 25% remains.
Writing that all radiation is equally penetrating and equally harmful in every situation.
Ignoring basic protection principles: time, distance and shielding.

20. Worked Examples

Worked Example 1: Alpha decay

Complete:

²³⁸₉₂U → ? + ⁴₂He

Mass number: 238 − 4 = 234. Atomic number: 92 − 2 = 90, which is thorium.

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

Worked Example 2: Convert Curie to Becquerel

For 2.0 Ci:

2.0 × 3.7 × 10¹⁰ = 7.4 × 10¹⁰ Bq

Worked Example 3: Half-life fraction

A radioactive sample passes through three half-lives.

Remaining fraction = (1/2)³ = 1/8 = 12.5%

Worked Example 4: Radiocarbon dating

A sample has 1/8 of its original C-14 activity. Three half-lives have elapsed.

Age ≈ 3 × 5730 = 17,190 years

Worked Example 5: Identify the process

A heavy nucleus absorbs a neutron and splits into smaller nuclei while releasing more neutrons.

Answer: nuclear fission.

21. Important Exam Questions

Short-Answer Questions

Define nuclear chemistry.
Define radioactivity.
Differentiate chemical and nuclear reactions.
Define natural radioactivity with one example.
Define artificial radioactivity with one example.
Compare alpha, beta and gamma rays.
What happens to A and Z during alpha decay?
What happens to A and Z during beta-minus decay?
What is the SI unit of radioactivity?
Define one becquerel.
Write the relation between Curie and Becquerel.
Define nuclear reaction.
State two conservation rules used to balance nuclear equations.
Define nuclear fission.
Define nuclear fusion.
Write one fission equation.
Write one fusion equation.
Differentiate fission and fusion.
What is a nuclear chain reaction?
How is nuclear power generated?
State any three industrial uses of radioactivity.
State any three medical uses of radioactivity.
What is radiocarbon dating?
What is the half-life of carbon-14?
Why does C-14 content decrease after an organism dies?
State four harmful effects of nuclear radiation.
State the three basic radiation-protection principles.

Long-Answer Questions

Explain natural and artificial radioactivity with examples.
Compare the properties of alpha, beta and gamma radiations.
Explain units of radioactivity and their conversions.
Explain nuclear reactions and balancing of nuclear equations.
Describe nuclear fission and the chain-reaction concept.
Describe nuclear fusion and state where it occurs naturally.
Distinguish between nuclear fission and fusion.
Write a descriptive note on nuclear power generation.
Distinguish nuclear power from nuclear weapons at a descriptive level.
Explain industrial applications of radioisotopes.
Explain medical applications of radioisotopes.
Explain the principle of radiocarbon dating.
Describe harmful effects of nuclear radiation and basic protection measures.

Numerical / Equation Practice

Balance alpha-decay nuclear equations.
Balance beta-decay nuclear equations.
Convert Bq to Ci and Ci to Bq.
Find remaining radioactive fraction after 1–4 half-lives.
Estimate simple radiocarbon age from a remaining fraction such as 1/2, 1/4 or 1/8.

Diagram Questions

Draw chemical vs nuclear change.
Draw natural vs artificial radioactivity.
Draw alpha, beta and gamma penetrating-power comparison.
Draw a nuclear-equation balance diagram.
Draw a fission chain-reaction concept.
Draw a fusion reaction concept.
Draw the nuclear-power energy-conversion flow.
Draw industrial applications of radioactivity.
Draw the radiocarbon-dating cycle.
Draw the radiation-damage pathway.

Exam Strategy This is a short 2-hour unit. Memorize the chapter in this order: radioactivity → units → nuclear equation → fission/fusion → power → applications → C-14 dating → harmful effects. Keep at least one balanced equation ready for alpha decay, fission, fusion and C-14 decay.

22. One-Minute Revision

Nuclear chemistry studies changes in atomic nuclei.
Radioactivity is spontaneous decay of unstable nuclei.
Natural radioactivity occurs in naturally unstable isotopes.
Artificial radioactivity is produced by creating a radioactive nucleus through a nuclear reaction.
Alpha particle = ⁴₂He nucleus.
Alpha decay: A decreases by 4, Z decreases by 2.
Beta-minus decay: A unchanged, Z increases by 1.
Gamma emission changes nuclear energy but not A or Z.
Penetration: γ > β > α.
General ionizing power: α > β > γ.
1 Bq = 1 disintegration per second.
1 Ci = 3.7 × 10¹⁰ Bq.
1 Rutherford = 10⁶ Bq.
Nuclear equations must balance both A and Z.
Fission splits a heavy nucleus.
Fusion combines light nuclei.
Fission can sustain a neutron chain reaction.
Fusion powers the Sun and stars.
Nuclear power uses controlled fission to produce heat and electricity.
Nuclear weapons release nuclear energy destructively and are not controlled power systems.
Industrial uses include radiography, gauges and tracers.
Medical uses include diagnostic imaging and radiotherapy.
Carbon-14 is produced from atmospheric nitrogen.
C-14 half-life ≈ 5730 years.
After 1 half-life 50% remains; after 2, 25%; after 3, 12.5%.
Ionizing radiation can damage cells and DNA.
Radiation protection: minimize time, maximize distance, use proper shielding.

23. Diagram Practice

Students should practice these labelled diagrams for the NEB examination:

Chemical vs nuclear reaction.
Natural vs artificial radioactivity.
Penetrating power of α, β and γ.
Balancing mass number and atomic number.
Nuclear fission chain-reaction concept.
Nuclear fusion concept.
Nuclear-power generation flow.
Industrial applications of radioisotopes.
Radiocarbon-dating concept.
Radiation-damage pathway.

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