Radioactivity, alpha and beta and gamma decay, nuclear equations, half-life, nuclear fission and fusion, radiation safety, and the role of nuclear chemistry in medicine, energy, and research
Every chemical reaction you have studied so far — combustion, neutralization, precipitation, oxidation-reduction — involves the rearrangement of electrons between atoms. The nuclei of atoms sit untouched at the center while electrons shift, bond, and break bonds around them. Chemical reactions transform substances, but they leave the identity of each element intact.
Nuclear chemistry is different in a fundamental way. Nuclear changes happen inside the nucleus itself. Protons and neutrons rearrange. The identity of elements can change. The energies involved are millions of times larger than those released in ordinary chemical reactions. And the particles and radiation released by nuclear changes can pass through matter, damage living tissue, generate electricity, diagnose disease, and — in their most extreme applications — level cities.
Nuclear chemistry is not science fiction. It is the science behind smoke detectors, cancer treatment machines, the power plants that supply electricity to millions of homes, the geological clocks that tell us the age of ancient rocks and fossils, and the reactions that power the sun itself.
Understanding nuclear chemistry requires returning to the nucleus — the dense, positively charged core of every atom you first studied in Chapter 5 and revisited in Chapter 6. You already know that the nucleus contains protons and neutrons, that the atomic number identifies the element, and that isotopes are atoms of the same element with different numbers of neutrons. In this chapter, those earlier ideas become the foundation for something new: the chemistry of nuclear transformation.
A wooden artifact recovered from an ancient site contains only one-quarter of the carbon-14 found in a living tree. The half-life of carbon-14 is 5,730 years. How old is the artifact — and how can scientists possibly know that, given that no one was there to start a stopwatch when the tree was cut down? By the end of this chapter, you will be able to answer this question using the same reasoning that lets geologists date rocks billions of years old and doctors choose isotopes that disappear from the body within hours.
When iron rusts, it reacts with oxygen to form iron(III) oxide. The iron atoms are still iron. The oxygen atoms are still oxygen. The reaction rearranges electrons and forms new bonds, but no iron nucleus becomes something other than iron.
Nuclear chemistry is the study of changes that occur within the nucleus itself. In a nuclear change, the number of protons in an atom's nucleus can change — and when the number of protons changes, the element changes. An atom of carbon can become an atom of nitrogen. An atom of uranium can split into atoms of krypton and barium. An atom of hydrogen can fuse with another hydrogen atom to produce helium.
These are not chemical reactions in the ordinary sense. They are nuclear reactions.
| Aspect | Chemical Reactions | Nuclear Reactions |
|---|---|---|
| What changes | Electrons rearrange between atoms; bonds break and form. Elements keep their identity. | The nucleus itself changes; protons and neutrons are gained, lost, or rearranged. New elements are often created. |
| Energy scale | Hundreds of kilojoules per mole. | Millions of electron volts (MeV) per event — millions of times larger. |
| What triggers it | Temperature, concentration, catalysts, and the presence of reactants. | Instability inside the nucleus (proton-to-neutron ratio) or bombardment by particles. Largely unaffected by temperature, pressure, or chemical state. |
| Conservation laws | Atoms are conserved — same atoms before and after, rearranged. | Mass number and total charge are conserved, but a small amount of mass converts to energy: E = mc². |
Not all nuclei are created equal. Some nuclei are stable and remain unchanged indefinitely. Others are unstable — they have an unfavorable ratio of protons to neutrons or are simply too large to hold together. Unstable nuclei undergo radioactive decay: they emit particles or radiation and transform into a more stable nucleus.
Two forces compete inside every nucleus: the electrostatic repulsion between positively charged protons, which pushes protons apart, and the strong nuclear force, which holds protons and neutrons together at very short range. When these forces fall out of balance, the nucleus is unstable.
Every element with an atomic number above 83 (bismuth) is radioactive — these nuclei are simply too large for the strong nuclear force to hold together indefinitely. Many lighter elements have radioactive isotopes as well.
Nuclear reactions have been happening since the universe began. Fusion inside the sun converts hydrogen into helium and sustains life on Earth. The radioactive decay of uranium, thorium, and potassium inside Earth generates the heat that drives plate tectonics. The age of a rock or fossil determined by radiometric dating reflects billions of years of nuclear reactions inside individual atoms. Humans only learned to detect radioactivity in the late nineteenth century — but the reactions themselves are as old as matter.
In 1896, the French physicist Henri Becquerel was studying materials that glow after exposure to sunlight, a property called fluorescence. He placed a uranium compound on top of a photographic plate wrapped in black paper and left it in a drawer on a cloudy day, expecting nothing because there had been no sunlight to energize the uranium. When he developed the plate, he found a clear image — the uranium had exposed the film even in the dark.
Becquerel had discovered that uranium spontaneously emits radiation without any external energy source. This radiation came from inside the atoms themselves.
Marie Curie and her husband Pierre Curie studied this phenomenon intensely. Marie coined the word radioactivity to describe the spontaneous emission of radiation from unstable nuclei. By systematically separating uranium ore and identifying which fractions were most intensely radioactive, she discovered two new radioactive elements: polonium (named after her home country of Poland) and radium. Marie Curie won two Nobel Prizes — in Physics (1903) and Chemistry (1911) — and remains one of the most important scientists in history.
Ernest Rutherford — the same physicist who discovered the nuclear model of the atom through his gold foil experiment (Chapter 5) — investigated the types of radiation emitted by radioactive materials. He found the radiation was not all the same: some types were easily stopped, some traveled much farther, and some carried no charge at all. Rutherford named them alpha, beta, and gamma radiation — names still used today.
Radioactivity — the spontaneous emission of particles or electromagnetic radiation from an unstable nucleus.
Parent nucleus — the atom undergoing radioactive decay.
Daughter nucleus — the atom produced after decay. The daughter nucleus is always more stable than the parent — decay always moves a nucleus toward greater stability.
| Property of Radioactive Decay | What It Means |
|---|---|
| Spontaneous | Requires no external trigger, energy input, or chemical change. An unstable nucleus decays on its own schedule. |
| Random at the individual level | No instrument can predict when any single atom will decay. The average rate for a large collection of atoms, however, is extremely predictable (Section 26.5). |
| Unaffected by ordinary conditions | Heating, cooling, compressing, dissolving, or chemically reacting a radioactive substance does not change its decay rate. Stability depends on the nucleus, not the chemical environment. |
| A source of measurable radiation | The particles and energy released can be detected by Geiger counters, scintillation detectors, and photographic film — the same principle Becquerel discovered by accident. |
Radioactive nuclei decay by emitting different types of radiation. The three most important at this level are alpha radiation, beta radiation, and gamma radiation. They differ in what they consist of, how far they travel, how much ionizing power they have, and how they are stopped.
An alpha particle is identical to a helium-4 nucleus: it contains 2 protons and 2 neutrons, carries a charge of +2, and has a mass number of 4. When a nucleus emits an alpha particle, it loses 2 protons and 2 neutrons — the atomic number decreases by 2, the mass number decreases by 4, and the element changes.
Alpha particles are the largest and most massive type of radiation. Because of their size and charge, they interact strongly with matter and lose their energy quickly. Alpha particles can be stopped by a single sheet of paper, a few centimeters of air, or the outermost layer of human skin.
Alpha particles are not dangerous when the source is outside the body — your skin stops them before they reach living tissue. However, alpha emitters that are inhaled, ingested, or injected into the body can cause severe internal damage, because the tissue is then directly in the path of the particles.
A beta particle is a high-energy electron emitted from the nucleus. Beta particles carry a charge of −1 and have essentially no mass compared to protons and neutrons.
How does an electron come from the nucleus, when electrons orbit outside it? Inside the nucleus, a neutron converts into a proton and an electron. The electron is immediately ejected at high speed as a beta particle, while the proton remains in the nucleus — increasing the atomic number by 1 while the mass number stays the same.
Beta particles are smaller and faster than alpha particles. They can be stopped by a few millimeters of aluminum, a thin sheet of plastic, or dense materials such as lead or thick glass. Beta particles are more penetrating than alpha particles and can pass through skin to reach underlying tissue, so external exposure to strong beta sources requires shielding.
Gamma rays are high-energy electromagnetic radiation with no mass and no charge — part of the same electromagnetic spectrum as visible light and X-rays, but carrying far more energy. Gamma rays often accompany alpha or beta decay: when the daughter nucleus is left in an excited (higher-energy) state, it releases the extra energy as a gamma ray.
Because gamma rays have no charge and no mass, they interact weakly with matter and are highly penetrating. Gamma rays require substantial shielding — thick lead plates, dense concrete walls, or several meters of water — and can pass completely through the human body. External gamma exposure is the most significant hazard for people at a distance from a source.
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Composition | 2 protons, 2 neutrons (helium-4 nucleus) | 1 high-energy electron | High-energy electromagnetic radiation |
| Charge | +2 | −1 | 0 |
| Mass number | 4 | 0 | 0 |
| Penetrating power | Low | Medium | High |
| Stopped by | Paper, skin | Aluminum sheet | Thick lead or concrete |
| Ionizing power | Very high | Moderate | Low |
Notice that ionizing power and penetrating power move in opposite directions. Alpha particles are the most ionizing — they damage tissue intensely in a short range before stopping. Gamma rays are the most penetrating — they travel far, but each individual interaction is less intensely damaging.
Nuclear equations describe radioactive decay the same way chemical equations describe chemical reactions. They show what changes — but instead of atoms and molecules, nuclear equations show nuclei and particles.
Every nucleus in a nuclear equation is written using the nuclear symbol notation introduced in Chapter 6: the chemical symbol of the element is written in the center, the mass number (protons + neutrons) is written as a superscript to the upper left, and the atomic number (number of protons) is written as a subscript to the lower left.
For example, the nucleus of uranium-238 is written: 23892U
| Particle | Symbol | Atomic Number | Mass Number |
|---|---|---|---|
| Alpha particle | 42He (or 42α) | 2 | 4 |
| Beta particle | 0−1e (or 0−1β) | −1 | 0 |
| Gamma ray | 00γ | 0 | 0 |
| Proton | 11p | 1 | 1 |
| Neutron | 10n | 0 | 1 |
Nuclear equations must be balanced, just like chemical equations. Two quantities must be conserved on both sides of the equation:
1. The sum of the mass numbers must be equal on both sides.
2. The sum of the atomic numbers must be equal on both sides.
These two rules let you identify unknown products and verify that a nuclear equation is correctly written.
In alpha decay, the parent nucleus emits an alpha particle (42He). The daughter nucleus has a mass number 4 less than the parent and an atomic number 2 less than the parent.
23892U → 42He + ?Mass number: 238 − 4 = 234
Atomic number: 92 − 2 = 90 → thorium (Th)
22688Ra → 42He + ?Mass number: 226 − 4 = 222
Atomic number: 88 − 2 = 86 → radon (Rn)
In beta decay, a neutron in the nucleus converts to a proton and an electron. The electron (0−1e) is emitted. The daughter nucleus has the same mass number as the parent, but its atomic number increases by 1.
146C → 0−1e + ?Mass number: 14 − 0 = 14
Atomic number: 6 − (−1) = 7 → nitrogen (N)
13153I → 0−1e + ?Mass number: 131
Atomic number: 53 + 1 = 54 → xenon (Xe)
Gamma rays are often emitted alongside alpha or beta particles. Because a gamma ray has no mass and no charge, it does not change the mass number or atomic number of the nucleus — it only releases energy. A gamma symbol is simply added to the right side of the equation.
6027Co → 0−1e + 6028Ni + 00γ23592U + 10n → 9038Sr + ? + 2 10nMass number: 235 + 1 = 90 + ? + 2(1) → ? = 236 − 90 − 2 = 144
Atomic number: 92 + 0 = 38 + ? + 2(0) → ? = 92 − 38 = 54 → xenon (Xe)
23090Th).3215P).24195Am), the isotope found in smoke detectors.23290Th → 42He + ?
2411Na → 0−1e + ?
? → 42He + 22286Rn23592U + 10n → 14454Xe + ? + 2 10n31H), a radioactive isotope of hydrogen.21482Pb) undergoes beta decay. Write this equation and identify the daughter nucleus.You cannot predict when any individual radioactive atom will decay. Nuclear decay is random at the atomic level — a fact confirmed by decades of careful measurement. However, you can make precise predictions about a large collection of radioactive atoms.
The half-life of a radioactive isotope is the time it takes for exactly one-half of a sample of that isotope to decay.
After one half-life, half the original atoms remain. After two half-lives, half of that half remains — one-quarter of the original. After three, one-eighth. After four, one-sixteenth. This pattern is a geometric decrease, sometimes called exponential decay.
| Number of Half-Lives Elapsed | 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% |
| 5 | 1/32 | 3.125% |
Different radioactive isotopes have wildly different half-lives — some decay in fractions of a second; others persist for billions of years. The half-life is a fixed property of each isotope. It does not change based on how much of the substance is present, what temperature it is, or what chemical compound it is part of.
| Isotope | Half-Life | Significance |
|---|---|---|
| Polonium-214 | 0.000164 seconds | Extremely unstable |
| Iodine-131 | 8.02 days | Used in thyroid treatment |
| Radon-222 | 3.82 days | Naturally occurring indoor hazard |
| Cobalt-60 | 5.27 years | Used in cancer radiation therapy |
| Carbon-14 | 5,730 years | Used in radiocarbon dating |
| Radium-226 | 1,600 years | Historically studied by Marie Curie |
| Uranium-235 | 703.8 million years | Nuclear fission fuel |
| Uranium-238 | 4.47 billion years | Geological dating, nuclear fuel |
| Potassium-40 | 1.25 billion years | Geological dating |
32.08 days ÷ 8.02 days/half-life = 4.00 half-lives80.0 g → 40.0 g → 20.0 g → 10.0 g → 5.0 g60 min ÷ 15 min/half-life = 4 half-lives6.4 × 10⁶ ÷ 2⁴ = 6.4 × 10⁶ ÷ 16 = 4.0 × 10⁵ atoms5 half-lives = 45 years → 1 half-life = 9 yearsOne of the most elegant applications of half-life is radiocarbon dating, used to determine the age of organic materials such as wood, bone, cloth, and charcoal.
Carbon-14 (146C) is a radioactive isotope of carbon with a half-life of 5,730 years. It is continuously produced in the upper atmosphere when cosmic ray neutrons collide with nitrogen-14:
¹⁴N + 10n → 146C + 11p (written: 147N + 10n → 146C + 11p)
Carbon-14 is absorbed by plants during photosynthesis and enters the food chain. As long as a living organism is alive, the ratio of carbon-14 to carbon-12 in its tissues remains roughly constant because new carbon-14 is continuously incorporated.
When the organism dies, it stops taking in new carbon, and the carbon-14 it contains begins to decay by beta emission:
146C → 0−1e + 147N
By measuring what fraction of the original carbon-14 remains in a sample, scientists can calculate how many half-lives have elapsed since the organism died, and therefore how old it is.
Radiocarbon dating is reliable for samples up to about 50,000 years old — after roughly nine half-lives, too little carbon-14 remains to measure precisely. For older geological materials, scientists use isotopes with much longer half-lives, such as potassium-40 (1.25 billion years) and uranium-238 (4.47 billion years).
| Days Elapsed | Half-Lives Elapsed | Mass Remaining (mg) |
|---|---|---|
| 0 | 0 | 200.0 |
| 8.02 | 1 | ? |
| 16.04 | 2 | ? |
| 24.06 | 3 | ? |
| 32.08 | 4 | ? |
| 40.10 | 5 | ? |
Nuclear fission is the splitting of a large, heavy nucleus into two smaller nuclei, releasing a tremendous amount of energy and several neutrons.
Fission does not happen spontaneously at a significant rate under ordinary conditions. It is typically induced by bombarding a heavy nucleus with a neutron. The neutron is absorbed, creating an unstable intermediate that immediately splits.
23592U + 10n → 14156Ba + 9236Kr + 3 10n + energy
A uranium-235 nucleus absorbs a neutron, becomes unstable, and splits into barium-141 and krypton-92, releasing three neutrons and a large amount of energy. The exact products can vary — uranium-235 does not always split the same way — but neutrons and energy are always released.
The key to nuclear fission's power is the chain reaction. Each fission event releases 2 to 3 neutrons. Those neutrons can each strike another uranium-235 nucleus, triggering more fission events, which release more neutrons, which trigger still more events. If the chain reaction is uncontrolled, the number of fission events multiplies explosively in a fraction of a second.
For a chain reaction to be self-sustaining, there must be enough fissile material in a compact enough shape that the released neutrons are likely to hit other nuclei before escaping. The minimum amount of material required for a self-sustaining chain reaction is called the critical mass.
Controlled chain reactions happen in nuclear reactors. Control rods made of neutron-absorbing materials (typically boron or cadmium) are inserted into the reactor core to slow or stop the reaction. By adjusting the position of control rods, operators control the reaction rate and the heat generated.
Uncontrolled chain reactions are the basis of nuclear weapons, where fissile material is rapidly assembled above critical mass.
In a nuclear power plant, the fission chain reaction produces heat. That heat is used to boil water and produce steam. The steam drives turbines, which generate electricity — the same basic process used in coal or natural gas power plants, but with a nuclear reaction as the heat source instead of combustion.
A typical nuclear reactor uses uranium fuel in which the concentration of uranium-235 has been increased (enriched) from its natural abundance of about 0.7% to roughly 3–5%. This level of enrichment is not sufficient to produce a weapon — it is only enough to sustain a controlled reaction.
Nuclear power plants do not emit carbon dioxide during operation. However, they produce radioactive waste — spent fuel rods containing fission products and unreacted uranium. Safe long-term storage of this waste is an ongoing challenge in nuclear energy policy.
Where does all this energy come from? Albert Einstein's equation E = mc² (energy equals mass times the speed of light squared) provides the answer. In a nuclear reaction, a tiny amount of mass is converted directly into energy. The speed of light (c) is approximately 3 × 10⁸ m/s. Because c² is an enormous number, even a tiny decrease in mass produces an enormous amount of energy.
In a fission reaction, the total mass of the products is slightly less than the total mass of the reactants. That difference in mass — called the mass defect — is the source of the energy released.
This is not magic. Mass and energy are two forms of the same thing. Nuclear chemistry reveals this in a way that no ordinary chemical reaction does.
Nuclear fusion is the opposite of fission: it is the combining of two small, light nuclei to form a single larger nucleus, releasing enormous amounts of energy.
The most common fusion reactions involve isotopes of hydrogen: deuterium (21H), hydrogen with 1 proton and 1 neutron, and tritium (31H), hydrogen with 1 proton and 2 neutrons.
The most studied fusion reaction for energy production is:
21H + 31H → 42He + 10n + energy
Two hydrogen nuclei fuse to form a helium-4 nucleus and a neutron, releasing an enormous amount of energy.
For light elements, fusing two nuclei creates a product nucleus that is more tightly bound than either of the originals. The resulting nucleus has less total mass than the two separate nuclei that combined to form it. That mass difference is released as energy. The energy released per unit mass of fuel is even greater than for fission — fusion is more energetic than fission, gram for gram.
Fusion requires overcoming the electrostatic repulsion between two positively charged nuclei. To get two nuclei close enough for the strong nuclear force to take over and fuse them, they must be moving extremely fast — meaning they must be at an extremely high temperature. The temperatures required for sustained nuclear fusion are on the order of tens of millions of degrees Celsius — the temperatures found inside stars.
The sun is powered almost entirely by a series of fusion reactions called the proton-proton chain. In the sun's core, where temperatures exceed 15 million degrees Celsius, hydrogen nuclei fuse through several steps to produce helium-4, releasing energy as light and heat. The sun converts approximately 600 million metric tons of hydrogen into helium every second, releasing the energy that sustains all life on Earth.
Achieving controlled fusion for power generation on Earth has been one of the greatest challenges in physics and engineering. Researchers are developing two main approaches.
In magnetic confinement fusion, plasma (a superheated gas of nuclei and electrons) is contained by powerful magnetic fields in a donut-shaped reactor called a tokamak. The plasma is heated to fusion temperatures and, if the process succeeds, sustains a fusion reaction that releases more energy than was used to heat it.
In inertial confinement fusion, small pellets of hydrogen fuel are compressed and heated simultaneously by powerful lasers from all sides, briefly achieving fusion conditions.
Neither approach has yet achieved sustained net energy gain — producing more energy than is consumed in running the reactor — though significant progress was made in 2022 when the National Ignition Facility in California achieved fusion ignition for the first time. Fusion power remains one of the most promising and most technically difficult frontiers in energy research.
| Feature | Fission | Fusion |
|---|---|---|
| Process | Splitting heavy nuclei | Joining light nuclei |
| Fuel | Uranium-235, Plutonium-239 | Hydrogen isotopes (deuterium, tritium) |
| Energy released | Very large | Even larger per unit mass |
| Radioactive waste | Yes — significant | Much less; neutron activation of reactor materials |
| Technical challenges | Controlled chain reactions, waste storage | Achieving and sustaining extreme temperatures |
| Currently used for power? | Yes | Not yet at commercial scale |
| Natural occurrence | Rarely in nature | Inside stars |
Radiation cannot be seen, heard, smelled, or felt at low levels. This makes it frightening to many people. But radiation is also one of the most carefully studied and precisely measurable of all environmental hazards. The science of radiation safety — called health physics — provides clear guidelines for working with radioactive materials safely.
Understanding radiation safety requires thinking clearly about types of radiation, dose, exposure time, distance, and shielding.
Becquerel (Bq) — measures the activity of a radioactive source: how many decay events occur per second. One becquerel equals one decay per second.
Gray (Gy) — measures absorbed dose: the amount of radiation energy deposited per kilogram of tissue.
Sievert (Sv) — measures effective dose: absorbed dose weighted for the biological damage potential of the radiation type. Alpha particles, for example, receive a higher weighting factor than gamma rays because they are more biologically damaging per unit of absorbed energy. The sievert is the most important unit for assessing health risk.
For reference, the average annual radiation dose received by a person in the United States from natural and medical sources is approximately 6 to 7 millisieverts (mSv) per year.
Everyone is constantly exposed to low levels of radiation from natural sources, called background radiation.
| Source | Description |
|---|---|
| Cosmic radiation | High-energy particles from outer space passing through the atmosphere. Exposure is higher at higher altitude — frequent flyers and people at high elevation receive somewhat more. |
| Terrestrial radiation | Naturally occurring radioactive isotopes (uranium, thorium, potassium-40) present in trace amounts in soil, rock, and building materials. |
| Radon gas | A colorless, odorless, radioactive gas produced by the decay of uranium in soil and rock. Radon can accumulate in basements and poorly ventilated buildings, and is the second leading cause of lung cancer in the United States after smoking. |
| Internal radiation | Small amounts of radioactive potassium-40 and carbon-14 are present in every living organism, including the human body. |
| Medical sources | X-rays, CT scans, and nuclear medicine procedures contribute significantly to the average person's total radiation dose. |
1. Time — The less time spent near a radiation source, the lower the total dose received. Dose is proportional to exposure time.
2. Distance — Moving away from a radiation source reduces exposure rapidly. For a point source, dose decreases with the square of the distance (the inverse square law): double the distance, receive one-quarter the dose.
3. Shielding — Placing material between yourself and a source reduces exposure. The shielding required depends on the radiation type: alpha particles are stopped by paper; beta particles by thin metal; gamma rays require thick lead or concrete.
The biological effects of radiation depend on total dose, dose rate, the type of radiation, and the tissues exposed.
Acute radiation exposure occurs when a large dose is received over a short time. Very high doses (above several sieverts in hours) can cause radiation sickness — nausea, vomiting, hair loss, and damage to bone marrow and the immune system — and can be fatal. Acute exposure at these levels is rare and generally occurs only in nuclear accidents or very close proximity to unshielded sources.
Chronic radiation exposure occurs when a person receives small doses over a long period. Low-level chronic exposure from background radiation is experienced by everyone and is not associated with measurable health effects at the doses involved. Elevated chronic doses — such as those received by certain radiation workers before modern safety standards — can increase cancer risk over a lifetime, but the additional risk from occupational exposure at modern permitted levels is quite small.
Health physicists use the linear no-threshold (LNT) model to estimate cancer risk from low levels of radiation. This model assumes that any amount of radiation, no matter how small, carries some small risk, and that risk increases proportionally with dose. The LNT model is a conservative, protective assumption used for regulatory purposes — it likely overestimates risk at very low doses, but this is considered acceptable in a safety context because it leads to protective policies.
In laboratories, medical facilities, and nuclear power plants, radioactive materials are handled with shielding appropriate to the radiation type (lead containers, thick glass, concrete walls), remote-handling tools that let technicians work without direct contact, personal dosimeters that measure cumulative exposure over time, regular monitoring of workspaces for contamination, strict dose limits for workers over defined time periods, and careful disposal of radioactive waste according to regulations based on activity and half-life. Radioactive materials used in medicine — such as iodine-131 for thyroid treatment — are handled under strict protocols in dedicated facilities.
Scenario 1: A laboratory technician works with iodine-131 samples for thyroid research. The isotope emits beta particles and gamma rays. Her work requires standing 0.5 meters from the source for two hours each day.
Scenario 2: A homeowner in a region with granite bedrock is concerned about radon gas in her basement. Radon-222 undergoes alpha decay with a half-life of 3.82 days.
Scenario 3: A patient receives a PET scan using fluorine-18 (half-life = 109.8 minutes).
Nuclear medicine uses radioactive isotopes to image the inside of the body and diagnose disease with remarkable precision. These techniques depend on detecting gamma radiation emitted from inside the body after a patient is given a tiny amount of a radioactive tracer.
PET scanning (Positron Emission Tomography) uses an isotope that emits positrons — positively charged particles with the same mass as electrons. A common tracer is fluorine-18 attached to a glucose molecule. Cancer cells and active brain tissue take up glucose rapidly, so areas of high activity accumulate more tracer. The positrons annihilate with nearby electrons, producing pairs of gamma rays that travel in opposite directions and are detected simultaneously by the scanner, producing a detailed image of metabolic activity.
SPECT scanning (Single Photon Emission Computed Tomography) uses gamma-emitting isotopes such as technetium-99m (the "m" stands for metastable — it emits gamma rays without changing its atomic number). Technetium-99m is the most widely used radioactive isotope in medicine, with a half-life of about 6 hours — short enough to decay quickly after the procedure, minimizing patient exposure.
Thyroid scanning and iodine uptake studies use iodine-131 or iodine-123. The thyroid gland naturally concentrates iodine from the blood, so radioactive iodine given to a patient goes to the thyroid, and its distribution can be imaged to detect abnormal tissue or assess thyroid function.
Radioiodine therapy (iodine-131) is used to treat thyroid cancer and hyperthyroidism. The thyroid gland absorbs iodine-131, which then emits beta particles that destroy overactive or cancerous thyroid tissue from within. Because the thyroid is the primary tissue that concentrates iodine, the radiation is largely delivered to the target tissue.
External beam radiation therapy uses powerful gamma-ray sources — historically cobalt-60 machines, now often linear accelerators — to direct beams of radiation at tumor tissue from outside the body. By rotating the beam around the patient and focusing it on the tumor from multiple angles, medical physicists maximize dose to the tumor while minimizing dose to surrounding healthy tissue.
Brachytherapy involves placing small radioactive seeds or wires directly inside or adjacent to a tumor. Seeds containing iodine-125 or palladium-103 are commonly used for prostate cancer treatment. The radiation travels only a short distance, concentrating the dose in the tumor.
Nuclear power plants currently supply about 10% of the world's electricity and about 20% of the electricity generated in the United States. Countries such as France obtain approximately 70% of their electricity from nuclear power.
Nuclear power plants operate on the principle of controlled fission described in Section 26.6. Modern reactor designs include multiple passive safety features that operate without active intervention if the reactor loses power or control. Common designs — pressurized water reactors (PWRs) and boiling water reactors (BWRs) — use ordinary water as both moderator (slowing neutrons to speeds most likely to cause fission) and coolant (carrying heat away from the core).
Nuclear waste is the main environmental challenge of nuclear power. Spent fuel rods remain intensely radioactive and must be stored safely for thousands of years. Most spent fuel is currently stored on-site at nuclear plants in deep pools of water (providing both cooling and shielding) or in dry cask storage after cooling. Permanent geological repositories — underground storage sites in stable rock formations — have been studied extensively but have not been fully implemented in most countries.
| Method | Isotope (Half-Life) | Useful Age Range |
|---|---|---|
| Radiocarbon dating | Carbon-14 (5,730 years) | About 100 to 50,000 years — used for organic materials such as manuscripts, artifacts, remains, and wood |
| Potassium-argon dating | Potassium-40 → argon-40 (1.25 billion years) | About 100,000 years to billions of years — argon gas accumulates after rock solidifies, so the K-40/Ar-40 ratio reveals when the rock formed |
| Uranium-lead dating | Uranium-238 → lead-206 (4.47 billion years); Uranium-235 → lead-207 (703.8 million years) | Used together for cross-checking; the oldest Earth rocks (~4.0 billion years) and meteorites (~4.56 billion years, the age of the solar system) are dated this way |
Radioactive isotopes serve as tracers in biochemical and biological research — labeled molecules in which an atom has been replaced with a radioactive isotope and can be followed through a chemical or biological system by detecting its radiation.
For example, plants grown in an atmosphere containing radioactive carbon dioxide (containing carbon-14) incorporate radioactive carbon into their photosynthetic products. By tracking where the radioactive carbon appears in the plant, researchers mapped out the detailed biochemical pathway of photosynthesis — a landmark achievement in biochemistry. Radioactive tracers have also been used to study kidney function, blood flow, enzyme mechanisms, drug metabolism, and molecular cell biology.
To find the fraction of a sample remaining after a whole number of half-lives:
Fraction remaining = (1/2)ⁿ
To find the number of half-lives elapsed:
n = total time elapsed ÷ half-life
To find the amount remaining:
Amount remaining = original amount × (1/2)ⁿ
Conservation rules for nuclear equations: the sum of mass numbers on the left equals the sum of mass numbers on the right, and the sum of atomic numbers on the left equals the sum of atomic numbers on the right.
21084Po → 42He + ?Mass number: 210 − 4 = 206
Atomic number: 84 − 2 = 82 → lead (Pb)
9038Sr → 0−1e + ?Mass number: 90 − 0 = 90
Atomic number: 38 − (−1) = 39 → yttrium (Y)
n = 21.08 ÷ 5.27 = 4.00 half-lives40.0 g → 20.0 g → 10.0 g → 5.0 g → 2.5 g? → 0−1e + 22688RaMass number: ? = 0 + 226 = 226
Atomic number: ? = −1 + 88 = 87 → francium (Fr)
Answer each question in two to four sentences.
This association comes from decades of science fiction and early watch faces painted with radioactive paint. In reality, radioactive materials do not glow on their own. Some materials that glow green, such as certain phosphorescent paints, are not radioactive at all. Radiation is invisible to the human eye.
This is not possible. A nuclear weapon requires highly enriched fissile material assembled to achieve an uncontrolled chain reaction in microseconds. Nuclear power plants use fuel enriched to about 3–5% uranium-235 — far too little for a weapon. The worst-case accident at a nuclear plant (such as Chernobyl) involves a steam explosion and fire, releasing radioactive material — serious and dangerous, but fundamentally different from a nuclear detonation.
All people receive background radiation constantly from natural sources, and low-level exposure is not acutely harmful. Health risk from radiation depends heavily on total dose. The doses received during diagnostic medical procedures, from living at high altitude, or even from eating a banana (which contains a small amount of potassium-40), do not present measurable health risks. The risks of high-dose exposure are real and serious, but they do not apply to the low doses encountered in everyday life.
Each radioactive isotope has a fixed half-life. Short-lived isotopes (days to years) become essentially inert relatively quickly. Long-lived isotopes (thousands to millions of years) do remain radioactive for geologically long periods, which is why permanent storage is challenging — but "forever" is not accurate. Every radioactive nucleus eventually decays to a stable daughter.
Fusion produces far less long-lived radioactive waste than fission, but it is not entirely free of radiation concerns. The high-energy neutrons produced by fusion reactions activate structural materials of the reactor, creating radioactive waste that requires careful handling. Fusion is a potentially cleaner energy source than fission, but not perfectly clean.
23892U → 42He + 23490Th. Verify that this equation is balanced by checking mass numbers and atomic numbers.22286Rn).13755Cs).147N + 10n → 146C + ?Chapter 26 explored changes that occur inside the nucleus — radioactivity, decay, half-life, fission, fusion, and the role of nuclear processes in medicine, energy, and research. Chapter 27 connects chemistry to the large-scale systems of civilization: water treatment, air quality, agriculture, fuels, plastics, manufacturing, and public health. You have studied matter, reactions, energy, and nuclear changes in depth. Now you will see how chemical knowledge is applied at the scale of communities, nations, and the planet.
In Chapter 27, you will learn how drinking water is made safe through chemical treatment, how air pollutants form and are monitored, how nitrogen chemistry underlies modern agriculture, how combustion products affect the atmosphere, how plastics are made and what happens to them at the end of their useful life, and how chemists and engineers assess risk, measure exposure, and make decisions about chemical hazards in the environment. Chemistry does not happen only in laboratories — it happens in rivers and aquifers, in urban air, in agricultural fields, in industrial plants, and in every home.