Radioactive decay, half-life, fission and fusion made simple
HESI A2 topic: Nuclear chemistry
- Alpha decay: mass number drops 4, atomic number drops 2.
- Beta decay: a neutron becomes a proton, so atomic number rises 1.
- Paper stops alpha, thin metal stops beta; thick lead only reduces gamma.
- Each half-life halves what is left: ½, then ¼, then ⅛.
- Fission splits heavy nuclei (reactors); fusion joins light ones (the sun).
Key facts
- Isotopes of an element have the same number of protons (atomic number) but different mass numbers.
- Alpha decay: the nucleus loses 2 protons and 2 neutrons, so the mass number drops by 4 and the atomic number drops by 2.
- Beta decay: a neutron becomes a proton and an electron is emitted, so the atomic number rises by 1 and the mass number is unchanged.
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- Gamma rays are high-energy electromagnetic radiation with no charge; they do not change the mass number or atomic number.
- Penetrating power: alpha is stopped by paper or the outer layer of skin, beta by a thin layer of metal; gamma is very penetrating and is best reduced by thick, dense material such as lead.
- After n half-lives, the fraction left is (1/2)ⁿ: one-half, one-quarter, one-eighth, and so on.
- Fission splits a heavy nucleus such as uranium-235 and powers nuclear reactors; fusion joins light nuclei and powers the sun.
Quick check1 question · 2 more at the end
What is an alpha particle made of?
Correct answer: A.
An alpha particle is a helium nucleus: two protons and two neutrons. When a nucleus emits one, its mass number drops by 4 and its atomic number drops by 2. Alpha particles are the least penetrating type of radiation and are stopped by paper.
A Correct: an alpha particle is a helium nucleus with two protons and two neutrons.
B A high-energy electron from the nucleus is a beta particle.
C High-energy electromagnetic radiation describes gamma rays, which have no charge and do not change the mass number.
D One proton and one electron make up an ordinary hydrogen atom, not an alpha particle.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Unstable nuclei decay by giving off radiation. Alpha particles (2 protons, 2 neutrons) are stopped by paper, beta particles (electrons) by thin metal, and gamma rays are only cut down by thick, dense shielding such as lead. A half-life is the time for half of a sample to decay. Fission splits heavy nuclei; fusion joins light ones.
1 What makes a nucleus radioactive?A nucleus is radioactive when its mix of protons and neutrons is unstable, so it breaks down toward a more stable form. 1 min
A nucleus is radioactive when its mix of protons and neutrons is unstable, so it breaks down toward a more stable form. Protons and neutrons, together called nucleons, are held in the nucleus by the strong nuclear force. Nuclei with too many or too few neutrons for their protons fall outside the stable range and decay.
Isotopes are atoms of the same element (same number of protons) with different mass numbers. Carbon-14 is written ¹⁴₆C: mass number 14 on top, atomic number 6 below.
2 What are alpha, beta, and gamma radiation?Each type of radiation changes the nucleus in its own way. 1 min
Each type of radiation changes the nucleus in its own way. Keep track of the mass number (A) and atomic number (Z).
| Radiation | What it is | Charge | Change in A | Change in Z |
|---|---|---|---|---|
| Alpha (α) | Helium nucleus: 2 protons + 2 neutrons | Positive | −4 | −2 |
| Beta (β) | High-energy electron from the nucleus | Negative | 0 | +1 |
| Gamma (γ) | High-energy electromagnetic radiation | None | 0 | 0 |
In beta decay, a neutron turns into a proton and an electron is shot out. The extra proton is why the atomic number goes up by one, and why the atom becomes a different element. Gamma rays are given off when an excited nucleus drops to a lower energy state, and they can accompany alpha or beta decay.
3 How far does each type of radiation travel?Alpha is the easiest radiation to block, beta takes metal, and gamma is the hardest to stop. 1 min
Alpha is the easiest radiation to block, beta takes metal, and gamma is the hardest to stop.
| Radiation | Gets through | Blocked by |
|---|---|---|
| Alpha | Very little | Paper or the outer layer of skin |
| Beta | Paper, wood, a hand | A thin sheet of metal |
| Gamma | Thick layers of most materials | Only reduced; thick, dense lead works best |
Easy to block does not mean harmless. An alpha emitter outside the body does little damage, but breathed in or swallowed it is a serious hazard.
4 How do you solve half-life problems?A half-life is the time it takes for half of the radioactive atoms in a sample to decay. 1 min
A half-life is the time it takes for half of the radioactive atoms in a sample to decay. Each half-life cuts the remaining amount in half, no matter how much you start with.
- Find the number of half-lives: total time ÷ half-life.
- Halve the starting amount that many times.
Example: an isotope has a half-life of 6 hours. You start with 80 mg. After 18 hours, 18 ÷ 6 = 3 half-lives have passed: 80 → 40 → 20 → 10 mg remain.
To work backward, find how many halvings give the fraction left. If one-eighth remains, 3 half-lives have passed (½ × ½ × ½ = ⅛). Carbon-14, used to date ancient remains, has a half-life of 5,730 years.
5 What is the difference between fission and fusion?Both release huge amounts of energy, but in opposite ways. 1 min
Both release huge amounts of energy, but in opposite ways.
| Fission | Fusion | |
|---|---|---|
| What happens | A heavy nucleus splits into smaller ones | Very light nuclei join into a heavier one |
| Example | Uranium-235 absorbs a neutron and splits, releasing 2–3 more neutrons | Hydrogen nuclei fuse into helium |
| Where it happens | Nuclear power reactors | The sun |
| Key condition | Enough fuel (critical mass) for a chain reaction | Extremely high temperatures |
6 How is radiation used in medicine?Some radioactive isotopes collect in specific tissues, so doctors can detect the radiation they give off. 1 min
Some radioactive isotopes collect in specific tissues, so doctors can detect the radiation they give off. Compounds of technetium-99m (half-life about 6 hours) are taken up by certain damaged tissues in the heart, liver and lungs, and its gamma rays are used for imaging.
Iodine-131 concentrates in the thyroid and is used to monitor and treat thyroid conditions. Cobalt-60 gives off gamma rays that are used in radiation therapy for cancer.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Alpha particles are the least penetrating, but they are hazardous if an alpha source is inhaled or swallowed.
Watch out: A sample is not gone after two half-lives; one-quarter of it remains.
Watch out: Beta decay increases the atomic number; the emitted electron comes from a neutron changing into a proton, not from the electron shells.
Watch out: Fission splits; fusion fuses. The sun runs on fusion, and today's nuclear power plants run on fission.
How to remember it
Memory hook: "Paper, metal, lead." That is the barrier for alpha, beta and gamma, in Greek-letter order. For half-life, just keep halving: 1 → ½ → ¼ → ⅛.
Self-check:
- Uranium-238 (Z = 92) gives off an alpha particle. What are the new mass number and atomic number?
- 64 g of an isotope with a 2-day half-life sits for 8 days. How much is left?
- Which process powers the sun, and which one powers today's nuclear reactors?
Show answers
- Mass number 234, atomic number 90.
- Alpha decay removes 2 protons and 2 neutrons.
- Mass number: 238 − 4 = 234.
- Atomic number: 92 − 2 = 90.
- 4 g.
- Number of half-lives: 8 days ÷ 2 days = 4.
- Halve 4 times: 64 → 32 → 16 → 8 → 4 g.
- Fusion powers the sun; fission powers today's reactors.
- The sun fuses hydrogen nuclei into helium.
- Reactors split uranium-235 nuclei.