Radioactive decay is the process by which an unstable atomic nucleus changes into a more stable form by releasing particles or energy. It matters in chemistry because decay changes the identity, mass, or energy of atoms, which affects nuclear medicine, dating ancient materials, and radiation safety. The main types students study are alpha decay, beta minus decay, positron decay, and gamma decay.
Each type has a different emitted particle and a different effect on the nucleus.
Understanding Chemistry: Types of Radioactive Decay
Radioactivity begins with the balance of forces inside a nucleus. Protons repel each other because they have the same electric charge. The strong nuclear force holds protons and neutrons together at very short distances.
In large nuclei, or nuclei with an unsuitable neutron to proton balance, this holding force may not provide enough stability. Decay is one way for the nucleus to move toward a lower energy arrangement. This is not a chemical reaction.
Chemical reactions rearrange electrons around atoms. Nuclear decay changes the nucleus itself, so the energy involved is far greater and the element can change.
Alpha decay is common in very heavy nuclei, where releasing a tightly bound group of two protons and two neutrons reduces the crowding in the nucleus. Beta processes solve a different problem. A nucleus may contain too many neutrons or too many protons for its size.
In beta minus decay, a neutron changes into a proton through the weak nuclear force. In positron decay, a proton changes into a neutron. Tiny particles called neutrinos or antineutrinos leave at the same time.
They carry away part of the energy and momentum. They interact so weakly with matter that enormous numbers pass through Earth and through people without noticeable effects.
A nucleus can be stable in its proton and neutron numbers but still have extra energy. It may then release that energy as a gamma ray. Gamma emission often follows alpha or beta decay because the daughter nucleus is left in an excited state.
Gamma rays have no rest mass and no electric charge, yet they can be very penetrating. Radiation matters because it can ionize atoms in materials. Ionization can damage molecules in living cells, including DNA.
Alpha radiation is most dangerous when an alpha emitting substance enters the body. Gamma sources can be dangerous outside the body because their radiation travels farther.
Shielding is chosen for the radiation type. Paper can stop alpha particles, plastic or thin metal can reduce beta radiation, and dense materials such as lead or thick concrete reduce gamma radiation.
Decay is random for one particular nucleus. No one can predict the exact moment when it will decay. For a huge sample, though, the pattern is reliable.
The half life is the time needed for half of the undecayed nuclei in a sample to transform. This idea supports radiometric dating, medical tracers, and the safe storage of radioactive waste. When writing nuclear equations, keep separate counts for total nucleons and total positive nuclear charge.
First identify what changes in the parent nucleus. Then use the periodic table to find the daughter element.
Check that the equation describes a physically possible change, not merely numbers that appear to balance. This careful checking prevents common mistakes with beta particles, whose charge can make the element shift in a direction that seems surprising at first.
Key Facts
- Alpha decay emits an alpha particle, 4/2 He, so mass number decreases by 4 and atomic number decreases by 2.
- Beta minus decay emits an electron, 0/-1 e, and an antineutrino, so atomic number increases by 1 while mass number stays the same.
- Positron decay emits a positron, 0/+1 e, and a neutrino, so atomic number decreases by 1 while mass number stays the same.
- Gamma decay emits a gamma ray, γ, so atomic number and mass number do not change.
- Alpha penetration is low, beta and positron penetration are moderate, and gamma penetration is high.
- Nuclear equations must conserve mass number and atomic number on both sides.
Vocabulary
- Radioactive decay
- Radioactive decay is the spontaneous change of an unstable nucleus into a more stable nucleus by emitting particles or electromagnetic radiation.
- Alpha particle
- An alpha particle is a helium nucleus with 2 protons and 2 neutrons, written as 4/2 He.
- Beta particle
- A beta particle in beta minus decay is a high speed electron emitted when a neutron changes into a proton.
- Positron
- A positron is the antimatter partner of the electron and is emitted when a proton changes into a neutron.
- Gamma ray
- A gamma ray is high energy electromagnetic radiation released by an excited nucleus without changing the numbers of protons or neutrons.
Common Mistakes to Avoid
- Treating gamma decay as a change in element is wrong because gamma emission only lowers nuclear energy and does not change atomic number or mass number.
- Forgetting to conserve atomic number in nuclear equations is wrong because the total charge number must match on both sides of the equation.
- Saying beta minus decay decreases atomic number is wrong because a neutron becomes a proton, so the atomic number increases by 1.
- Confusing positron decay with beta minus decay is wrong because positron decay emits 0/+1 e and decreases atomic number by 1, while beta minus decay emits 0/-1 e and increases atomic number by 1.
Practice Questions
- 1 Uranium-238 undergoes alpha decay. Write the complete nuclear equation and identify the daughter nucleus.
- 2 Carbon-14 undergoes beta minus decay. What are the atomic number and mass number of the daughter nucleus, and what element is formed?
- 3 A radioactive sample emits radiation that is stopped by paper, another that passes through paper but is reduced by thin aluminum, and another that requires thick lead for strong shielding. Identify the likely decay radiation types and explain the reasoning.