Nuclear Chemistry
Grade 11 · Science · Study Guide
Explore radioactive decay, nuclear stability, half-life calculations, fission, fusion, and the applications of nuclear reactions from power plants to medicine.
Nuclear chemistry deals with changes in the nucleus of an atom — reactions that alter the number of protons or neutrons, releasing enormous amounts of energy. Unlike chemical reactions that involve valence electrons, nuclear reactions involve the strong nuclear force binding protons and neutrons (nucleons) in the nucleus. The energy released follows Einstein's equation E = mc², where even tiny mass changes produce tremendous energy.
Radioactive decay occurs when an unstable nucleus spontaneously emits particles or energy to reach a more stable configuration. The three major decay types are: alpha decay (emission of a ⁴₂He nucleus — two protons, two neutrons — reducing atomic number by 2 and mass number by 4); beta decay (a neutron converts to a proton, emitting a high-energy electron (β⁻) and increasing atomic number by 1); and gamma decay (emission of high-energy photons from an excited nucleus — no change in mass or atomic number). In all nuclear equations, mass numbers and atomic numbers are conserved on both sides.
Half-life (t₁/₂) is the time required for half of a radioactive sample to decay. It is a constant for a given isotope, unaffected by temperature, pressure, or chemical state. After n half-lives, the fraction remaining = (½)ⁿ. Carbon-14 (t₁/₂ ≈ 5,730 years) is used to date organic materials up to ~50,000 years old. Uranium-238 (t₁/₂ ≈ 4.5 billion years) is used to date ancient rocks.
Nuclear fission is the splitting of a heavy nucleus (e.g., U-235) into two smaller nuclei plus neutrons and energy. The released neutrons can trigger more fissions — a chain reaction. In a nuclear reactor, this is controlled to produce electricity. In a nuclear bomb, it is uncontrolled. Nuclear fusion is the combining of two light nuclei (e.g., H-2 + H-3 → He-4 + neutron) releasing even more energy than fission per unit mass. Fusion powers stars — including our Sun — and is the subject of ongoing research for clean energy.
| Decay Type | Particle/Ray Emitted | Change in Atomic Number (Z) | Change in Mass Number (A) | Penetrating Power |
|---|---|---|---|---|
| Alpha (α) | ⁴He nucleus | −2 | −4 | Low (stopped by paper) |
| Beta (β⁻) | Electron (e⁻) | +1 | 0 | Medium (stopped by aluminum) |
| Gamma (γ) | High-energy photon | 0 | 0 | High (needs lead/concrete) |
| Number of Half-Lives | Fraction Remaining | Percent Remaining |
|---|---|---|
| 0 | 1 | 100% |
| 1 | 1/2 | 50% |
| 2 | 1/4 | 25% |
| 3 | 1/8 | 12.5% |
| 4 | 1/16 | 6.25% |
| Property | Fission | Fusion |
|---|---|---|
| Process | Heavy nucleus splits | Light nuclei combine |
| Example | U-235 + n → Ba + Kr + 3n | H-2 + H-3 → He-4 + n |
| Energy released | Very large | Even larger per unit mass |
| Current use | Nuclear power plants, weapons | Stars; experimental reactors |
| Fuel | Uranium, Plutonium | Hydrogen isotopes (deuterium, tritium) |
Key terms
- Radioactive Decay
- Spontaneous emission of particles or energy from an unstable nucleus to reach a more stable state.
- Alpha Decay
- Emission of a ⁴He nucleus; mass number decreases by 4, atomic number by 2.
- Beta Decay
- A neutron converts to a proton, emitting an electron (β⁻); atomic number increases by 1.
- Gamma Decay
- Emission of high-energy photons from an excited nucleus; no change in mass or atomic number.
- Half-Life (t₁/₂)
- Time for half of a radioactive sample to decay; constant for a given isotope.
- Nuclear Fission
- Splitting of a heavy nucleus into lighter nuclei + neutrons + energy; used in nuclear reactors.
- Nuclear Fusion
- Combining two light nuclei into a heavier one, releasing tremendous energy; powers stars.
- Mass Defect
- Difference between the mass of a nucleus and the sum of its individual nucleons; converted to binding energy via E = mc².
Practice questions
An element undergoes alpha decay. What happens to its atomic number?
- Increases by 2
- Decreases by 2
- Increases by 4
- Stays the same
Show answer
An alpha particle is ⁴₂He — losing 2 protons decreases atomic number by 2 and mass number by 4.
A radioactive sample has a half-life of 10 years. What fraction remains after 30 years?
- 1/2
- 1/4
- 1/8
- 1/16
Show answer
30 years = 3 half-lives. Fraction remaining = (1/2)³ = 1/8.
In beta decay, a neutron converts to a proton, emitting a high-energy ______.
Show answer
Beta-minus (β⁻) decay emits an electron (and antineutrino). The neutron becomes a proton, increasing atomic number by 1.
Which type of nuclear reaction powers the Sun?
- Alpha decay
- Beta decay
- Nuclear fission
- Nuclear fusion
Show answer
The Sun fuses hydrogen nuclei (protons) into helium-4 via the proton-proton chain, releasing enormous energy as described by E = mc².
Gamma radiation is BEST described as:
- A stream of electrons
- A stream of helium nuclei
- High-energy electromagnetic radiation
- Slow-moving neutrons
Show answer
Gamma rays are high-frequency photons emitted from an excited nucleus. They have no mass or charge but high penetrating power.
Fun facts
- The energy released from 1 kg of uranium-235 in fission equals approximately 3 million kg of coal burned — the nuclear-to-chemical energy ratio is staggering.
- Radiocarbon dating revolutionized archaeology — Willard Libby won the 1960 Nobel Prize in Chemistry for developing the technique using C-14's 5,730-year half-life.
- The proton-proton chain fusion reaction in the Sun converts about 600 million tonnes of hydrogen into helium every second — yet the Sun has enough hydrogen fuel to continue for another 5 billion years.
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