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Chemistry Unit 9: Nuclear chemistry

Unstable nuclei decay by emitting radiation, change into other elements, and release energy in fission and fusion.

Covers: Radioactivity, alpha, beta and gamma radiation, balancing nuclear equations, half-life, fission and fusion, uses and risks of radioisotopes

Study guide

Nuclear chemistry studies changes in the nucleus. Most nuclei are stable, but some isotopes have unstable combinations of protons and neutrons and spontaneously decay, giving off radiation. These are radioisotopes. Natural transmutation occurs when one unstable nucleus decays on its own; artificial transmutation happens when a nucleus is bombarded with particles.

The main types of radiation differ in mass, charge and penetrating power. An alpha particle (⁴₂He) is two protons and two neutrons; it has the most mass and a +2 charge but the least penetrating power, stopped by paper or skin. A beta particle (⁰₋₁e) is a high-speed electron released when a neutron turns into a proton; it penetrates more but is stopped by thin metal. A positron (⁰₊₁e) has the electron's mass but a positive charge. Gamma rays (⁰₀γ) are high-energy electromagnetic radiation with no mass or charge and the greatest penetrating power, requiring thick lead or concrete. In an electric field, alpha particles bend toward the negative plate, beta particles toward the positive plate, and gamma rays don't bend. Nuclear equations must balance both mass numbers (top) and atomic numbers (bottom): in alpha decay, mass number drops by 4 and atomic number by 2; in beta decay, atomic number rises by 1 with no change in mass number.

Each radioisotope has a characteristic half-life, the time for half of a sample to decay. After n half-lives, the fraction remaining is (1/2)ⁿ. Half-life can't be changed by temperature or pressure. Fission splits a heavy nucleus, like uranium-235, into smaller nuclei, releasing large amounts of energy and neutrons that can cause a chain reaction; it powers nuclear reactors but produces long-lived radioactive waste. Fusion combines light nuclei, like hydrogen, into heavier ones, releasing even more energy per mass; it powers the sun but requires extremely high temperatures. In both, a small amount of mass is converted into energy (E = mc²), far more energy than chemical reactions release. Radioisotopes have uses: carbon-14 dates once-living material, uranium-238 dates rocks, iodine-131 diagnoses and treats thyroid disorders, cobalt-60 treats cancer and food irradiation kills bacteria. Risks include biological damage from radiation and the storage of nuclear waste.

Key ideas

• Unstable nuclei decay and emit radiation.
• Alpha is heaviest and least penetrating; gamma is massless and most penetrating.
• Nuclear equations balance mass numbers and atomic numbers.
• After n half-lives, (1/2)ⁿ of a sample remains.
• Fission splits heavy nuclei; fusion joins light nuclei; both convert mass into energy.

Formulas and rules

Common mistakes

• Forgetting to balance the atomic numbers as well as the mass numbers.
• Thinking heating a sample changes its half-life; it doesn't.

Words to know

Radioisotope

An unstable isotope that decays and emits radiation.

Transmutation

Changing one element into another by nuclear reaction.

Alpha particle

A helium nucleus with a +2 charge and low penetrating power.

Beta particle

A high-speed electron emitted from the nucleus.

Positron

A particle with the mass of an electron and a positive charge.

Gamma ray

High-energy electromagnetic radiation with no mass or charge.

Penetrating power

How far radiation can travel through matter.

Half-life

Time for half of a radioactive sample to decay.

Nuclear fission

Splitting a heavy nucleus into smaller nuclei.

Chain reaction

Neutrons from fission causing more fission.

Nuclear fusion

Combining light nuclei into a heavier nucleus.

Mass defect

Mass converted into energy in a nuclear reaction.

Carbon-14 dating

Using C-14 decay to date once-living materials.

Nuclear waste

Radioactive byproducts that must be stored safely.

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.