Nuclear Chemistry and Radioactivity
Grade 12 · Science · Study Guide
Explore the forces holding atomic nuclei together, types of radioactive decay, nuclear reactions, and real-world applications.
The nucleus of an atom is held together by the strong nuclear force, which overcomes the electromagnetic repulsion between positively charged protons. However, some nuclei are unstable and undergo radioactive decay — spontaneous emission of particles or energy to achieve a more stable configuration. There are three main types of radioactive decay: alpha (α) decay emits a helium-4 nucleus and reduces atomic number by 2 and mass number by 4; beta (β) decay converts a neutron to a proton (emitting an electron) or a proton to a neutron (emitting a positron); gamma (γ) decay releases high-energy electromagnetic radiation with no change in mass or atomic number.
Half-life is a crucial concept: it is the time required for half of a radioactive sample to decay. After one half-life, 50% remains; after two, 25%; after three, 12.5%; and so on. Half-lives range from fractions of a second to billions of years. Radiocarbon dating uses the predictable decay of carbon-14 (half-life ~5,730 years) to determine the age of organic materials.
Nuclear reactions release far more energy than chemical reactions because they involve changes in the nucleus itself — binding energy is converted to mass or vice versa according to Einstein's famous equation E = mc². Nuclear fission splits heavy nuclei (like uranium-235) releasing enormous energy used in nuclear power plants. Nuclear fusion combines light nuclei (like hydrogen isotopes) releasing even more energy — this powers the Sun and is the goal of fusion energy research.
| Type | Particle emitted | Change in Z | Change in A | Penetration |
|---|---|---|---|---|
| Alpha (α) | ⁴He nucleus | −2 | −4 | Low (stopped by paper) |
| Beta (β⁻) | Electron | +1 | 0 | Medium (stopped by aluminum) |
| Beta (β⁺) | Positron | −1 | 0 | Medium |
| Gamma (γ) | Photon | 0 | 0 | High (requires lead/concrete) |
Key terms
- Radioactive decay
- Spontaneous transformation of an unstable nucleus, emitting particles or energy.
- Half-life
- The time required for half of a radioactive sample to decay.
- Alpha (α) decay
- Emission of a helium-4 nucleus; reduces atomic number by 2 and mass number by 4.
- Beta (β) decay
- Conversion of neutron to proton (β⁻) or proton to neutron (β⁺) within the nucleus.
- Nuclear fission
- Splitting of a heavy nucleus into lighter nuclei, releasing large amounts of energy.
- Nuclear fusion
- Combining of light nuclei into a heavier nucleus, releasing even more energy per unit mass.
- Binding energy
- Energy required to completely separate all nucleons in a nucleus; reflects nuclear stability.
Flashcards
- Alpha decay
- Emits ⁴₂He; Z decreases by 2, A decreases by 4; stopped by paper
- Beta-minus decay
- Neutron → proton + electron; Z increases by 1, A unchanged
- Gamma decay
- Emits high-energy photon; no change in Z or A; requires lead shielding
- Half-life
- Time for half of radioactive nuclei to decay; independent of sample size or temperature
- Nuclear fission
- Heavy nucleus splits into smaller nuclei + neutrons + huge energy; used in nuclear reactors
- E = mc²
- Einstein's mass-energy equivalence; nuclear reactions convert tiny mass into enormous energy
Practice questions
Which type of radiation has the greatest penetrating power?
- Alpha particles
- Beta particles
- Gamma rays
- Neutrons
Show answer
Answer: Gamma rays
Gamma rays are high-energy electromagnetic radiation with no mass or charge, giving them the greatest penetrating power. They require dense materials like lead or thick concrete to stop.
In alpha decay, the atomic number decreases by 2.
Show answer
Answer: True
Alpha decay emits a ⁴₂He nucleus (2 protons, 2 neutrons), so the parent nucleus loses 2 protons (atomic number decreases by 2) and 4 nucleons (mass number decreases by 4).
The time for half of a radioactive sample to decay is called the ___.
Show answer
Answer: half-life
Half-life is a constant for each radioactive isotope. It does not depend on temperature, pressure, or the amount of sample present.
Which process powers the Sun?
- Nuclear fission
- Chemical combustion
- Nuclear fusion
- Alpha decay
Show answer
Answer: Nuclear fusion
The Sun fuses hydrogen nuclei (protons) into helium-4 through the proton-proton chain. This releases enormous energy because the helium nucleus has slightly less mass than the four protons, and the difference is converted to energy.
Gamma decay changes both the atomic number and mass number of the nucleus.
Show answer
Answer: False
Gamma decay only emits a photon (electromagnetic radiation). There is no change in atomic number (Z) or mass number (A) — the nucleus simply releases excess energy.
Fun facts
- Carbon-14 dating can only be used for organic materials up to about 50,000 years old — beyond that, too little C-14 remains to measure accurately.
- The energy released by 1 kg of uranium-235 in fission equals roughly 3 million kilograms of coal burned.
- Positron emission tomography (PET) scans in medicine use beta-plus decay to create images of metabolic activity inside the body.
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