Nuclear Chemistry
Unstable nuclei, the radiation they give off, and how we use it
Ordinary chemistry rearranges electrons; the nucleus of each atom comes through unchanged. Nuclear chemistry is about the nucleus itself: how an unstable nucleus breaks down, what it gives off, and how an atom of one element can become another. In this unit you will read isotope symbols, balance alpha, beta and gamma decays, work with half-life, compare fission and fusion, and see how radiation dates ancient bones and treats cancer.
This unit builds on protons, neutrons and atomic number from The Periodic Table. Try the decay balancer and the half-life simulator, then take the quiz.
The Nucleus and Isotopes
The nucleus holds protons and neutrons. The number of protons is the atomic number, and it decides which element the atom is.[1] Isotopes of an element have the same number of protons but different numbers of neutrons.[1]
A nuclide symbol shows both counts. The mass number A (protons + neutrons) goes at the top left and the atomic number Z at the bottom left: 146C is carbon-14, with 6 protons and 14 − 6 = 8 neutrons.
Why are some nuclei unstable? Forces inside the atom push it toward a strong, stable balance. A nucleus with excess energy gets rid of it by giving off radiation: that is radioactivity.[2] Carbon-12 never decays, but carbon-14 does.[1]
Alpha, Beta and Gamma Decay
- Alpha (α) particles are made of two protons and two neutrons,[3] the same as a helium-4 nucleus, 42He. The nucleus loses 4 from A and 2 from Z.
- Beta (β⁻) particles are small, fast-moving particles:[3] electrons, written 0−1e. The electron carries a charge of −1, so to keep the totals equal Z goes up by 1 and A stays the same.
- Gamma (γ) rays are packets of energy called photons.[3] They carry no mass or charge, so A and Z do not change; the nucleus just sheds energy.
In every nuclear equation the mass numbers add up to the same total on both sides, and so do the atomic numbers. Radon-222 is an alpha emitter:[6]
22286Rn → 21884Po + 42He
Check: 222 = 218 + 4, and 86 = 84 + 2. Atomic number 84 is polonium.
Carbon-14 decays into nitrogen-14,[4] which is a beta decay: A stays at 14 and Z rises from 6 to 7.
146C → 147N + 0−1e
What stops each kind? Alpha particles are blocked by a sheet of paper, your skin or a few inches of air, though they are dangerous if inhaled or swallowed. A thin sheet of metal or plastic or a block of wood stops beta particles. Gamma rays need several feet of concrete or a few inches of dense material such as lead.[2]
Decay Balancer
Choose a nuclide and a kind of decay. Predict the daughter's mass number and atomic number, then check.
22286Rn → ? + 42He
Radon-222 gives off an alpha particle (helium-4 nucleus) and becomes polonium-218: mass number 218, atomic number 84.
Radon-222 emits alpha particles.
Half-Life
A half-life is the time it takes half of the atoms of a radioactive isotope to decay.[2] Every radioactive isotope has its own fixed half-life,[3] ranging from about a minute for radon-220 (55.6 s) to billions of years for thorium-232 (about 14 billion years).[17]
After one half-life, 1/2 of the atoms remain; after two, 1/4; after n, (1/2)n. You cannot say when any single atom will decay; it is a matter of chance. With many atoms, the chances average out to a smooth curve.
| Isotope | Half-life | Use or source |
|---|---|---|
| Fluorine-18 | 110 minutes[7] | Medical imaging[7] |
| Radon-222 | 3.8 days[6] | Natural gas from uranium in rocks and soil[6] |
| Iodine-131 | 8.02 days[5] | Thyroid cancer treatment[5] |
| Carbon-14 | about 5,730 years[4] | Dating once-living things[4] |
Worked example: how much iodine-131 is left after 24 days?
- The half-life is about 8 days,[5] so 24 days is 24 ÷ 8 = 3 half-lives.
- (1/2)3 = 1/8. Start with 80 mg and 10 mg remain.
Half-Life Simulator
Start with 100 atoms. Each step is one half-life, and every atom has a 50% chance of decaying. Compare your random sample with the ideal curve.
100 atoms of carbon-14 at the start.
Half-lives: carbon-14,[4] iodine-131[5] and fluorine-18.[7] The shape of the curve is the same for every isotope; only the time on each step changes.
Fission and Fusion
Fission splits a heavy nucleus. Some nuclei split after absorbing an extra neutron, forming two new nuclei (fission products) and releasing usually two or three neutrons.[2] Those neutrons can split more nuclei: a chain reaction. Uranium-235 is the fuel in commercial nuclear power plants.[2] A reactor controls the chain reaction, and its heat makes steam that spins a turbine to generate electricity.[9]
Fusion joins light nuclei. Two light nuclei merge into a single heavier nucleus, and fusion reactions power the Sun and other stars.[8] Energy is released because the new nucleus has less mass than the two nuclei that formed it; the missing mass becomes energy.[8]
Medicine and Dating
- PET scans. Fluorine-18 emits positrons (β⁺). Each positron meets an electron and produces gamma photons that a scanner uses to make images.[7] Its 110-minute half-life[7] means it is mostly gone within a day.
- Radiotherapy. Radiation therapy uses high doses of radiation to kill cancer cells and shrink tumors; it works by damaging their DNA.[10] Gamma rays from cobalt-60 are often used.[2]
- Iodine-131. The thyroid gland absorbs iodine,[5] so I-131, a beta and gamma emitter, is often used to treat thyroid cancer.[5]
Carbon-14 dating. Carbon-14 forms in the atmosphere when cosmic radiation strikes it. While an organism lives, it keeps taking in carbon. After it dies, the supply stops and its carbon-14 declines at a fixed rate.[16] With a half-life of about 5,730 years, carbon-14 has dated bones and campfires as old as 60,000 years.[4]
Worked example: a bone has 1/4 of its original carbon-14. How old is it?
1/4 = (1/2)2, so 2 half-lives have passed: 2 × 5,730 = about 11,460 years.
Dose and Safety
Radiation is all around us. Americans receive about 620 millirem a year on average. Half comes from natural background radiation, mostly radon in the air, with smaller amounts from cosmic rays and the Earth itself. A chest x-ray gives about 10 millirem.[11]
Three rules cut your dose from any source:[12]
- Time. Spend less time near the source and your dose is lower.[12]
- Distance. The dose drops dramatically as you move farther away, just as a fire feels cooler from across the room.[12]
- Shielding. Barriers of lead, concrete or water protect against penetrating gamma rays.[12]
In a school lab, never handle a radioactive source with bare hands, keep it in its container when not in use, and follow your teacher's instructions.
How Do We Know?
Becquerel's photographic plates (1896)
In 1896 Henri Becquerel was studying how uranium salts respond to light. By accident he found that the salts give off a penetrating radiation on their own, strong enough to register on a photographic plate. It was not X-rays but something new: radioactivity.[13]
The Curies (1903 Nobel Prize)
Pierre Curie and Marie Curie researched the radiation Becquerel had discovered. In 1903 the three shared the Nobel Prize in Physics: half to Becquerel for discovering spontaneous radioactivity, half jointly to the Curies for their research.[14]
Rutherford: alpha and beta (1899)
In 1899 Ernest Rutherford showed that there were at least two distinct types of radiation: alpha and beta. In 1902 he and Frederick Soddy proposed that elements can disintegrate and turn into other elements, exactly what a decay equation shows.[15]
Libby's carbon clock (1960 Nobel Prize)
In 1949 Willard Libby developed a method for dating with carbon-14. He won the 1960 Nobel Prize in Chemistry “for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science.”[16]
Test Yourself
Ten questions. The balancer and simulator are allowed!
The Half-Life Challenge
Think About It
Questions to puzzle over in class or at home.
- Alpha particles are stopped by paper. Why are alpha emitters still dangerous if swallowed or breathed in?
- Why is a 110-minute half-life useful for a scan, but useless for dating a 10,000-year-old campfire?
- Your simulator run rarely lands exactly on the ideal curve. Why would 1,000,000 atoms follow it more closely than 100?
- Most background radiation comes from radon. Where might radon build up in a house, and why?
- Becquerel discovered radioactivity by accident. What did he need to notice to turn the accident into a discovery?
Sources
- U.S. Department of Energy, Office of Science, “DOE Explains…Isotopes.” Quotes: “Hydrogen and its two naturally occurring isotopes, deuterium and tritium.” “Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons.” “The number of protons in a nucleus determines the element’s atomic number on the Periodic Table.” “Carbon-12 is stable, meaning it never undergoes radioactive decay. Carbon-14 is unstable and undergoes radioactive decay with a half-life of about 5,730 years (meaning that after 5,730 years half of the material will have decayed to the stable isotope nitrogen-14).” https://www.energy.gov/science/doe-explainsisotopes
- U.S. Nuclear Regulatory Commission, “Radiation Basics.” Quotes: “These forces within the atom work toward a strong, stable balance by getting rid of excess atomic energy (radioactivity).” Alpha particles “can be blocked by a sheet of paper, skin, or even a few inches of air” and are “potentially dangerous if they are inhaled or swallowed”. “a thin sheet of metal or plastic or a block of wood can stop beta particles”. For gamma rays and x-rays: “Several feet of concrete or a few inches of dense material (such as lead) are able to block these types of radiation.” “a half-life of a radioactive material is the time it takes one-half of the atoms of a radioisotope to decay by emitting radiation”, ranging from “fractions of a second (for radon-220) to millions of years (for thorium-232)”. “In some elements, the nucleus can split as a result of absorbing an additional neutron, through a process called nuclear fission.” Fission brings the “release of neutrons (usually two or three), and formation of two new nuclei (fission products)”. Uranium-235 “is the isotope that is used as fuel in commercial nuclear power plants.” “gamma rays (such as from cobalt-60) are often used in medical applications to treat cancer”. https://www.nrc.gov/about-nrc/radiation/health-effects/radiation-basics.html
- U.S. Environmental Protection Agency, “Radioactive Decay.” Quotes: “Every radionuclide has a specific decay rate, which is measured in terms of “half-life”.” Alpha particles are “made up of two neutrons and two protons.” Beta particles are “small, fast-moving particles.” Gamma rays are “made up of weightless packets of energy called photons.” https://www.epa.gov/radiation/radioactive-decay
- National Institute of Standards and Technology, “How Do You Know the Age of Fossils and Other Old Things?” Quote: “As carbon-14 decays, with a half-life of about 5,730 years, it becomes nitrogen-14. Using this clock, they have dated bones, campfires and other objects as old as 60,000 years, and in some cases even older.” https://www.nist.gov/how-do-you-measure-it/how-do-you-know-age-fossils-and-other-old-things
- U.S. Environmental Protection Agency, “Radionuclide Basics: Iodine.” Quotes: “Type of Radiation Emitted: Beta Particles, weak Gamma Rays”. “Half-life: Iodine-131: 8.02 days”. “I-131 is often used to treat thyroid cancer.” “Once inside the body, radioactive iodine will be absorbed by the thyroid gland”. https://www.epa.gov/radiation/radionuclide-basics-iodine
- U.S. Environmental Protection Agency, “Radionuclide Basics: Radon.” Quotes: “Type of Radiation Emitted: Alpha Particles”. “Half-life: 3.8 days”. “It comes from the natural decay of uranium and radium found in nearly all rocks and soils.” https://www.epa.gov/radiation/radionuclide-basics-radon
- U.S. National Library of Medicine, DailyMed, “Fludeoxyglucose F-18 injection” (drug label). Quote: “Fluorine F 18 decays by positron (β+) emission and has a half life of 110 minutes. The principal photons useful for diagnostic imaging are the 511 keV gamma photons, resulting from the interaction of the emitted positron with an electron”. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0eb82924-1ce4-680e-e054-00144ff88e88
- U.S. Department of Energy, Office of Science, “DOE Explains…Fusion Reactions.” Quotes: “Depiction of the deuterium (D) and tritium (T) fusion reaction, which produces a helium nucleus (or alpha particle) and a high energy neutron.” “Fusion reactions power the Sun and other stars. In fusion, two light nuclei merge to form a single heavier nucleus. The process releases energy because the total mass of the resulting single nucleus is less than the mass of the two original nuclei.” https://www.energy.gov/science/doe-explainsnuclear-fusion-reactions
- U.S. Department of Energy, Office of Nuclear Energy, “Nuclear 101: How Does a Nuclear Reactor Work?” Quotes: Reactors “contain and control nuclear chain reactions that produce heat through a physical process called fission. That heat is used to make steam that spins a turbine to create electricity.” “Reactors use uranium for nuclear fuel.” https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work
- National Cancer Institute, “Radiation Therapy to Treat Cancer.” Quotes: “Radiation therapy (also called radiotherapy) is a cancer treatment that uses high doses of radiation to kill cancer cells and shrink tumors.” “Radiation therapy kills cancer cells or slows their growth by damaging their DNA.” https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy
- U.S. Nuclear Regulatory Commission, “Doses in Our Daily Lives.” Quotes: “On average, Americans receive a radiation dose of about 0.62 rem (620 millirem) each year. Half of this dose comes from natural background radiation. Most of this background exposure comes from radon in the air, with smaller amounts from cosmic rays and the Earth itself.” “a chest x-ray typically gives a dose of about 0.01 rem (10 millirem)”. https://www.nrc.gov/about-nrc/radiation/around-us/doses-daily-lives.html
- U.S. Environmental Protection Agency, “Protecting Yourself from Radiation.” Quotes: “Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time reduces the dose from the radiation source.” “Distance: The dose of radiation decreases dramatically as you increase your distance from the source, just as the heat from a fire reduces as you move further away.” “Shielding: Barriers of lead, concrete or water provide protection from penetrating gamma rays”. https://www.epa.gov/radiation/protecting-yourself-radiation
- NobelPrize.org, “Henri Becquerel – Facts.” Quote: “When Henri Becquerel investigated the newly discovered X-rays in 1896, it led to studies of how uranium salts are affected by light. By accident, he discovered that uranium salts spontaneously emit a penetrating radiation that can be registered on a photographic plate. Further studies made it clear that this radiation was something new and not X-ray radiation: he had discovered a new phenomenon, radioactivity.” https://www.nobelprize.org/prizes/physics/1903/becquerel/facts/
- NobelPrize.org, “The Nobel Prize in Physics 1903.” Quote: The prize “was divided, one half awarded to Antoine Henri Becquerel “in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity”, the other half jointly to Pierre Curie and Marie Curie, née Skłodowska “in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel””. https://www.nobelprize.org/prizes/physics/1903/summary/
- NobelPrize.org, “Ernest Rutherford – Facts” (Nobel Prize in Chemistry 1908). Quotes: “In 1899 Ernest Rutherford demonstrated that there were at least two distinct types of radiation: alpha radiation and beta radiation.” “In 1902 they formulated a revolutionary theory: that elements could disintegrate and be transformed into other elements.” Prize motivation: “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances”. https://www.nobelprize.org/prizes/chemistry/1908/rutherford/facts/
- NobelPrize.org, “Willard F. Libby – Facts” (Nobel Prize in Chemistry 1960). Quotes: Prize motivation “for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science”. “Carbon-14 forms in the atmosphere when acted upon by cosmic radiation”. “When an organism dies and the supply of carbon from the atmosphere ceases, the content of carbon-14 declines through radioactive decay at a fixed rate. In 1949 Willard Libby developed a method”. https://www.nobelprize.org/prizes/chemistry/1960/libby/facts/
- National Nuclear Data Center, Brookhaven National Laboratory, NuDat 3 decay data. Values: radon-220 ground state, half-life “55.6 s”, decay mode “α: 100 %”; thorium-232 ground state, half-life “14.0×109 y”, decay mode “α: 100 %”. https://www.nndc.bnl.gov/nudat3/