Radioactive decay happens when an unstable atomic nucleus changes into a more stable form by releasing particles or energy. The three major types are alpha decay, beta decay, and gamma decay. Each type sends out a different kind of radiation and changes the nucleus in a different way.
Understanding these decays helps explain nuclear energy, medical imaging, radiation safety, and how scientists date ancient materials.
Alpha decay releases a heavy particle made of two protons and two neutrons, so it strongly changes the element. Beta decay changes a neutron into a proton or a proton into a neutron, so it changes the atomic number by one. Gamma decay releases high-energy electromagnetic radiation, usually after another decay leaves the nucleus excited.
The main differences among alpha, beta, and gamma radiation are what is emitted, how far it penetrates matter, and how the nucleus changes.
Key Facts
- Alpha decay emits an alpha particle: 4/2 He, which has 2 protons and 2 neutrons.
- In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2: A/Z X -> A-4/Z-2 Y + 4/2 He.
- Beta minus decay emits an electron and an antineutrino: n -> p + e- + antineutrino.
- In beta minus decay, the atomic number increases by 1 while the mass number stays the same.
- Gamma decay emits a gamma ray photon: excited nucleus -> lower-energy nucleus + gamma.
- Penetrating power usually increases in the order alpha < beta < gamma, while ionizing power usually decreases in the order alpha > beta > gamma.
Vocabulary
- Radioactive decay
- Radioactive decay is the spontaneous change of an unstable atomic nucleus into a more stable nucleus by emitting particles or energy.
- Alpha particle
- An alpha particle is a helium nucleus made of two protons and two neutrons.
- Beta particle
- A beta particle is a high-speed electron or positron emitted during beta decay.
- Gamma ray
- A gamma ray is a high-energy photon emitted from an excited atomic nucleus.
- Penetrating power
- Penetrating power is the ability of radiation to pass through materials before being absorbed or stopped.
Common Mistakes to Avoid
- Confusing alpha particles with single protons is wrong because an alpha particle contains 2 protons and 2 neutrons, the same as a helium nucleus.
- Changing the mass number during beta decay is wrong because beta decay changes a neutron into a proton or a proton into a neutron, so the total number of nucleons stays the same.
- Thinking gamma decay changes the element is wrong because gamma emission releases energy but does not change the atomic number or mass number.
- Ranking alpha radiation as the most penetrating is wrong because alpha particles are large and highly charged, so paper or skin can often stop them.
Practice Questions
- 1 A uranium-238 nucleus undergoes alpha decay. What are the mass number and atomic number of the daughter nucleus?
- 2 Carbon-14 undergoes beta minus decay. If carbon has atomic number 6, what element is produced and what is its mass number?
- 3 A nucleus emits a gamma ray after beta decay. Explain why gamma emission changes the energy of the nucleus but not the identity of the element.