21. Nuclear Chemistry
🎯 Learning outcomes
After completing this chapter, you should be able to:
- Describe natural and artificial radioactivity.
- Give units of radioactivity.
- Explain nuclear reactions.
- Distinguish between nuclear fission and fusion reactions.
- Describe nuclear power and nuclear weapons.
- Explain industrial uses of radioactivity.
- State the medical uses of radioactivity.
- Explain radiocarbon dating.
- Describe the harmful effects of nuclear radiations.
1Nuclear reactions vs chemical reactions
The branch of chemistry that deals with changes of matter brought about by the nucleus of an atom, rather than by the electrons in the valence shell.
| Chemical reaction | Nuclear reaction |
|---|---|
| Elements are neither created nor destroyed. | New elements are formed from the parent elements. |
| Mostly valence electrons are involved. | Mostly nuclei are involved. |
| Rate depends on temperature, pressure, catalyst, etc. | Spontaneous; not affected by temperature, pressure, catalyst, etc. |
| Small amount of energy released or absorbed. | Huge amount of energy released or absorbed. |
2Natural and artificial radioactivity
| Natural radioactivity | Artificial radioactivity |
|---|---|
| Spontaneous emission of radiation from unstable nuclei. | Emission of radiation from naturally occurring nuclei when they are bombarded with high-energy sub-atomic particles, X-rays or γ-rays. |
| Elements usually have long half-lives. | Elements usually have short half-lives. |
| Usually occurs in elements with atomic number greater than 83, which are naturally unstable. | Can be induced even in elements with low atomic numbers. |
23892U → 23490Th + 42He | 147N + 42He → 189F → 178O + 11H |
Isotopes of an element whose nuclei are unstable and emit radiation are called radioisotopes (e.g. carbon-14, iodine-131, cobalt-60).
3Units of radioactivity
Radioactivity is measured as the number of disintegrations per unit time made by a radioactive substance from its unit mass.
Rate of disintegration of 1 g of radium-226 = 3.7 × 1010 disintegrations per second.
1 Bq = activity of 1 disintegration per second.
4Nuclear fission and fusion
A reaction in which one nucleus is converted into another new nucleus — one element is converted into another element.
Nuclear fission
A process in which a large nucleus splits into smaller nuclei.
Fission of uranium produces two energetic neutrons which initiate a chain reaction that completes within a short span of time, releasing a large amount of energy and raising the temperature of the surroundings by about a million degrees. This is nuclear energy.
Controlled using a moderator; used for commercial power generation.
Chain reaction left to run freely; used in nuclear weapons.
Nuclear fusion
A nuclear reaction in which lighter nuclei fuse to form a heavier nucleus.
Fusion is not easy: an extremely large amount of energy is needed to force two light nuclei together against nuclear repulsion. The millions of degrees required can only be obtained from nuclear fission. The hydrogen bomb explosion is a fusion reaction.
Fusion is also called a thermonuclear reaction because it needs extremely high temperatures (millions of degrees) to occur.
5Nuclear power and nuclear weapons
Nuclear power
- Application of nuclear reactions to generate electricity.
- Heat from a nuclear reactor turns water into steam, which rotates a steam turbine and generates electricity.
- Provides about 15% of the world’s electricity (estimated).
- Sustainable source of energy
- Reduces carbon emissions
- Expensive to establish a plant
- Needs high technology and qualified manpower
- Causes a nuclear threat to people
- Radioactive waste disposal is a serious issue
Nuclear weapons
- Devices that release energy explosively through nuclear fission, fusion, or both.
- Cause catastrophic effects by blast, fire and lethal ionising radiation.
- On 6 August 1945 the US dropped Little Boy (uranium fission bomb, 64 kg) on Hiroshima, and on 9 August 1945 Fat Man (plutonium implosion-type fission bomb) on Nagasaki. Nearly 200,000 people were killed.
- Nuclear weapons are held by the US, Russia, the UK, France, China, India and North Korea.
e.g. uranium-235, plutonium-239. Based on uncontrolled chain reactions.
Mainly fusion of hydrogen isotopes: deuterium and tritium.
6Industrial and medical uses
Industrial uses
- Measuring the thickness of materials
- Checking for blocked water pipes and leaking oil pipes
- Studying cracks, wear and tear in machines
- Sterilising materials
- Generating electricity
Tracing reaction mechanisms
Radioactive isotopes label atoms so their path in a reaction can be followed. In photosynthesis, is the O2 released from CO2 or from H2O? Label the oxygen with 18O (*O):
6CO*2 + 6H2O → C6H12O*6 + 6O*2
Both glucose and O2 contain 18O.
6CO2 + 6H2O* → C6H12O6 + 6O*2
Only O2 contains 18O.
O atoms from CO2 form both glucose and O2; O atoms from H2O form only O2.
Chemical synthesis and agriculture
- Synthesis: intense γ-rays from cobalt-60 on a mixture of ethene and hydrogen bromide at room temperature give bromoethane: H2C=CH2 + HBr → CH3CH2Br
- Agriculture: fertilisers labelled with radioisotopes allow metabolism in plants (and animals) to be studied.
Medical uses
Iodine-131 locates malignant cells of the thyroid gland.
Iodine-131 also destroys malignant thyroid cells. Cobalt-60 is used in cancer therapy and food irradiation.
7Radiocarbon dating
Carbon dating estimates the age of archaeological materials such as fossils, bone, wood, paper and leather.
How carbon-14 forms and decays
In the upper atmosphere, neutrons in cosmic rays bombard nitrogen:
C-14 is weakly radioactive and decays continuously by emitting β rays:
Living plants absorb 14CO2 during photosynthesis, and it then enters animals. In all living plants and animals, 14C has the same decay rate: 15.3 β rays per minute per gram of natural carbon. When they die, the carbon cycle exchange stops, but 14C keeps decaying, so its activity falls steadily.
Calculating the age
Compare the β-ray activity of the dead sample with that of living material:
Use the half-life your syllabus or question gives (commonly about 5730 years; this chapter uses 5760 years). Use the same time unit for t and λ.
8Harmful effects of nuclear radiation
- Nuclear radiations (α, β, γ, n) have enough ionising power to affect atoms of living cells, causing tissue damage and alteration of DNA / genes.
- High exposure may cause cancer — yet the right dose (γ-rays from Co-60) also destroys cancerous cells.
- High-level exposure may cause radiation sickness (nausea, vomiting, even death).
- Children and fetuses are more sensitive to radiation.
- Genetic effects may not be visible immediately.
- Short- or long-term exposure harms both animals and vegetation.
Exam tips
- Do not say nuclear reactions depend on temperature or catalysts — they do not.
- Balance both mass number (top) and atomic number (bottom) in nuclear equations.
- Fission = splitting a heavy nucleus; fusion = joining light nuclei (needs millions of degrees).
- SI unit of radioactivity is the becquerel, not the curie.
Quick summary
- Chemical vs nuclear: electrons vs nuclei; small vs huge energy changes.
- Natural = spontaneous, long half-life; artificial = induced by bombardment.
- 1 Ci = 3.7 × 1010 Bq; 1 Bq = 1 disintegration per second.
- Fission: chain reaction; controlled (power) or uncontrolled (atomic bomb).
- Fusion: hydrogen isotopes → helium; hydrogen bomb.
- Uses: tracers, I-131 (thyroid), Co-60 (cancer therapy, sterilisation), C-14 dating.
- Dating: t = (2.303/λ) log(N0/N); λ = 0.693/t½.
- Hazards: DNA damage, cancer, radiation sickness, genetic effects.