Nuclear chemistry is about what happens inside the nucleus, where unstable atoms decay and nuclei can split or fuse, releasing far more energy per gram of material than chemical reactions do. After this unit, you should be able to balance nuclear equations, compare types of radiation, explain fission and fusion, and solve half-life problems.
Read the big ideas first, then work through each topic's notes and try the checkpoint question before moving on.
Most chemical reactions involve changes in electron arrangement while the nuclei stay the same. This unit focuses on the nucleus, where protons and neutrons can rearrange and one element can change into another.
Honors Chemistry Unit 13 covers nuclear chemistry. You learn the main types of radioactive decay (alpha, beta, gamma, plus positron emission and electron capture) and how to write and balance nuclear equations by conserving mass number and atomic number. You also learn how ionizing radiation affects living tissue and how it's used in medicine and industry, and how fission and fusion release energy through . The unit ends with half-life, the time it takes for half of a radioactive sample to decay, which you'll use in calculations and in radiometric dating like carbon-14.
Each decay mode changes the nucleus by a set amount. Alpha emission lowers the mass number by 4 and the atomic number by 2, beta-minus emission raises the atomic number by 1, and gamma emission changes neither. Once the decay mode is given, you can use these rules to identify the daughter nuclide.
Ionizing radiation can remove electrons from atoms and damage DNA, which makes high doses harmful. Controlled doses can treat cancer or sterilize supplies. In medical imaging, detectors locate radiation emitted by a tracer to show where it travels in the body.
In fission and fusion, the products have slightly less total mass than the reactants. That mass difference becomes energy through . Per gram of fuel, a nuclear reaction releases millions of times more energy than burning coal or gasoline.
In nuclear equations, total mass number and electric charge balance. Many spontaneous nuclear changes release energy because the products have less total rest mass, and a large sample decays at a rate set by the isotope's half-life.
Under ordinary conditions, temperature and pressure have almost no effect on decay rates. Chemical environment can slightly affect decay modes that involve atomic electrons, such as electron capture. If you can balance a nuclear equation and count half-lives, you can handle most problems in this unit.
Radioactivity happens when an unstable nucleus breaks down on its own and gives off particles or energy.
Alpha, beta, and gamma radiation are all ionizing.
Fission and fusion can release energy because the products have less total rest mass than the reactants.
A half-life is the time it takes for half of the radioactive nuclei in a sample to decay.
Radioactivity happens when an unstable nucleus breaks down on its own and gives off particles or energy. A nucleus is usually unstable because its neutron-to-proton ratio is off or because it is too large. No element with an atomic number greater than 82 has a stable isotope. Bismuth-209 is radioactive, but its half-life is extremely long.
The three decay types you'll use most change the nucleus in set ways. Alpha decay ejects , so the mass number drops by 4 and the atomic number drops by 2. Beta decay turns a neutron into a proton and ejects an electron, , so the atomic number rises by 1 and the mass number stays the same. Gamma emission releases a high-energy photon, , which changes neither number. Positron emission () and electron capture both lower the atomic number by 1.
Unlike a chemical reaction, a nuclear reaction can turn one element into another. This is called transmutation. To balance a nuclear equation, the top numbers (mass) must add up the same on both sides, and so must the bottom numbers (charge).
Worked example. Uranium-238 undergoes alpha decay. What forms? The mass number is . The atomic number is , which is thorium. So . To check, confirm that and .
Check yourself. Carbon-14 undergoes beta decay. What is the product? (Answer: Nitrogen-14. The mass number stays 14 and the atomic number goes from 6 to 7, so .)
| Radiation | Symbol | Change in mass number | Change in atomic number | Typical shielding |
|---|---|---|---|---|
| Alpha | $^{4}_{2}\text{He}$ | Decreases by 4 | Decreases by 2 | Paper, clothing, or the outer layer of skin |
| Beta | $^{0}_{-1}\text{e}$ | No change | Increases by 1 | Plastic or thin metal |
| Positron | $^{0}_{+1}\text{e}$ | No change | Decreases by 1 | Plastic or another low-Z material stops the positrons, then lead or concrete reduces the 511 keV gamma photons from annihilation |
| Gamma | $^{0}_{0}\gamma$ | No change | No change | Thick lead or concrete reduces exposure |
Alpha, beta, and gamma radiation are all ionizing. That means they carry enough energy to knock electrons off the atoms they hit. In living tissue, ionization can break chemical bonds, including bonds in DNA. Cells can usually repair damage from small doses. Large doses can cause radiation sickness, and long-term exposure raises cancer risk.
How dangerous a source is depends partly on where it is. Outside the body, the outer layer of skin stops alpha particles, while penetrating gamma rays can expose deeper tissue. An alpha emitter inside the body, such as inhaled radon gas, can cause severe localized damage because alpha particles deposit energy over a very short distance. The total risk still depends on the isotope's activity, energy, location, and the dose received. Absorbed dose is measured in grays (Gy), while equivalent and effective dose are measured in sieverts (Sv) to account for biological effects. A typical person in the U.S. gets about 3 mSv per year from natural background sources.
The same damage that makes radiation risky also makes it useful. Radiotherapy targets tumors because fast-dividing cancer cells are more vulnerable to DNA damage. Technetium-99m (half-life about 6 hours) is a common imaging tracer. Iodine-131 treats thyroid disease because the thyroid absorbs iodine. Gamma rays from cobalt-60 can sterilize sealed medical supplies and reduce pathogens or insects in food. This irradiation does not make the food radioactive.
Check yourself. Why is external exposure to a contained alpha source usually less dangerous than taking an alpha emitter into the body? (Answer: The outer layer of skin stops alpha particles. Inside the body, alpha particles can deposit a large amount of energy in nearby living tissue. You should still never handle a radioactive source without proper controls.)
Fission and fusion can release energy because the products have less total rest mass than the reactants. That decrease in rest mass becomes released energy through , and since is enormous, a tiny mass change means a huge amount of energy. A nucleus also has a mass defect, which is the difference between its actual mass and the total mass of its separate protons and neutrons.
Fission splits a heavy nucleus. A slow neutron hits uranium-235, which splits into two mid-sized fragments plus 2 or 3 neutrons. Those neutrons split more nuclei in a chain reaction. Control rods of boron or cadmium absorb extra neutrons to keep it steady, and the heat makes steam that spins a turbine.
Fusion joins light nuclei. In deuterium-tritium fusion, deuterium and tritium form helium-4 and a neutron. Because both nuclei are positive and repel, practical D-T systems use plasma temperatures around 100 million K and must confine the plasma long enough for enough reactions to occur. Deuterium is abundant, but tritium is scarce and would need to be bred from lithium. No fusion power plant currently supplies electricity to the grid.
Worked example. Find in . The left side has mass . The fragments add to , so neutrons. The atomic numbers check out too, since .
Check yourself. What do control rods do in a fission reactor? (Answer: They absorb neutrons, so fewer neutrons are available to cause more fissions. Pushing them in slows the reaction, and pulling them out speeds it up.)
| Feature | Fission | Fusion |
|---|---|---|
| What happens | A heavy nucleus splits into smaller ones | Light nuclei join into a heavier one |
| Typical fuel | Uranium-235, plutonium-239 | Deuterium and tritium (hydrogen isotopes) |
| Conditions needed | A neutron can trigger a single fission. A sustained chain reaction needs suitable neutron energies and a critical configuration of fuel | Very high temperature plus enough particle density and confinement time |
| Where it happens now | Nuclear power plants | Stars, including the sun, and experimental reactors |
| Waste | Fission products and transuranic elements; some remain hazardous for thousands of years or longer | Helium, tritium-handling waste, and neutron-activated reactor materials; less long-lived high-level waste is expected than from fission |
A half-life is the time it takes for half of the radioactive nuclei in a sample to decay. Each isotope has its own half-life. Iodine-131 is about 8 days, carbon-14 is about 5,730 years, and uranium-238 is about 4.5 billion years. Ordinary changes in temperature, pressure, and chemical bonding have almost no effect on half-life, though electron capture can shift very slightly with chemical environment.
Decay is exponential, not linear. After one half-life, 1/2 of the sample remains, after two 1/4 remains, and after three 1/8 remains. The general formula is , where is the number of half-lives. Half-life describes large groups of atoms, so you can't predict when any single atom will decay.
Radiocarbon dating relies on this. While an organism is alive, it exchanges carbon with its environment and keeps a carbon-14 ratio close to that of its surroundings. After death, its carbon-14 decays. Measuring what remains can estimate the age of once-living material up to about 50,000 years old.
Worked example. A sample contains 80.0 mg of iodine-131 ( days). How much remains after 24 days? The number of half-lives is . The mass left is mg, so 70.0 mg has decayed.
Some honors classes also use with , which also works when the time isn't a whole number of half-lives.
Check yourself. A piece of wood has 25% of the carbon-14 found in living trees. About how old is it? (Answer: 25% is , so two half-lives have passed. That's years.)
| Term | Definition |
|---|---|
| Isotope | Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. |
| Radioactive decay | Radioactive decay is the spontaneous breakdown of an unstable nucleus, which releases particles or energy. |
| Alpha particle | An alpha particle is a helium-4 nucleus, $^{4}_{2}\text{He}$, with two protons and two neutrons. |
| Beta particle | A beta particle is a high-speed electron, $^{0}_{-1}\text{e}$, that a nucleus emits when one of its neutrons turns into a proton. |
| Gamma ray | A gamma ray is a high-energy photon released from a nucleus, and it changes neither the mass number nor the atomic number. |
| Nuclear fission | Nuclear fission is the splitting of a heavy nucleus into smaller nuclei, which releases energy and neutrons. |
| Nuclear fusion | Nuclear fusion is the joining of light nuclei into a heavier nucleus, which releases energy. |
| Half-life | A half-life is the time it takes for half of the radioactive nuclei in a sample to decay. |
| Mass defect | Mass defect is the difference between the mass of a bound nucleus and the total mass of its separate protons and neutrons, and it corresponds to energy through $E = mc^2$. |
If half is gone after one half-life, it's tempting to think the rest is gone after two. Each half-life removes half of what is left, not half of the original. After two half-lives 25% remains, and after three 12.5% remains. Count the half-lives and multiply by 1/2 once for each one.
Many errors come from checking the top numbers and forgetting the bottom ones. Both the mass numbers and the atomic numbers must add up the same on each side. Once you find the missing atomic number, use the periodic table to name the element.
Gamma rays penetrate farther, but penetration and biological effect are different. Alpha particles are blocked by the outer layer of skin, yet an internal alpha emitter can cause severe localized damage because alpha particles deposit energy over a short distance. For the same absorbed dose, alpha radiation generally has a greater biological effect than beta or gamma radiation.
Both release energy from a change in mass, so the names blur together. Fission means splitting a heavy nucleus like uranium-235, and it already powers reactors today. Fusion means joining light nuclei like hydrogen isotopes, which happens in the sun but not yet in power plants.