Radioactive decay is the process by which an unstable nucleus loses energy by emitting radiation. This process allows a nucleus to reach a more stable state. The radiation emitted by an unstable nucleus may be a particle or energy in the form of electromagnetic radiation.Certain isotopes are more stable than others. Their stability is determined by the ratio of the number of neutrons to the number of protons in the nucleus. If the ratios of neutrons to protons for all elements are plotted as a graph, a nuclear zone of stability can be identified. The zone of stability in the graph shows that lighter isotopes (atomic mass number under 20) are most stable when the neutron-to-proton ratio is 1:1. For these elements, the repulsive force within the nucleus is not strong enough to overcome the attractive force that holds the nucleus together. The nucleus is unstable if the neutron-to-proton ratio is greater than 1.5. This instability is typical for very heavy elements. The nuclear zone of stability can be used to predict how unstable isotopes will decay.
Zone of Stability
There are three main types of radioactive decay:
- Alpha decay ( decay) is radioactive decay that gives off an particle. An particle is composed of two protons and two neutrons. It is essentially a helium nucleus.
- Beta decay ( decay) is radioactive decay that gives off a particle. A particle is a high-energy electron emitted from the nucleus rather than the electron cloud. In the nucleus, neutrons can decay into protons by emitting a particle.
- Gamma decay ( decay) is radioactive decay that gives off a gamma () ray. A ray is electromagnetic radiation with the shortest wavelength (), meaning it has the highest energy of all light.
There are other, less common types of radioactive decay. These include the emission of a positron (a nuclear particle with the mass of an electron but a charge of +1) and electron capture, in which the nucleus of an atom captures one of the atomic electrons and then emits a ray as well as an almost massless subatomic particle called a neutrino.
In and decay, the nature of the daughter nuclide is fundamentally changed compared to the parent nuclide. A chemical equation can be written to show the transmutation (conversion of one type of nucleus into another) that occurs with radioactive decay. By convention, the notation for a radioactive element is to write the mass number in superscript to the left of the element's atomic symbol. For example, a carbon-14 isotope can be shown as . Alternatively the atomic symbol followed by a dash and the mass number can be used (e.g., C-14). Note that carbon-14 is more unstable than carbon-12 and is therefore more likely to undergo radioactive decay. A carbon-12 isotope is more stable and is much less likely to undergo radioactive decay.For decay, the general equation is . The daughter nuclide is the element composed of two fewer protons than the parent nuclide—for example, thorium (Th) has two fewer protons than uranium (U). Thus, when undergoes decay, the equation shows that the mass number is decreased by 4 and the atomic number is decreased by 2.
A nuclear reaction that is related to positron emission is electron capture, in which the nucleus captures a high-energy electron. This transforms one proton into a neutron. The general formula for electron capture is . In electron capture, the atomic number and the identity of the element changes, but the mass number stays the same. Electron capture and positron emission are two ways through which a proton transforms to a neutron in the nucleus. Electron capture is technically not radioactive decay, as there is no particle emission.For decay, no general equation exists. This is because the makeup of the nucleus is not fundamentally changed—the emission is a high-energy photon, not a subatomic particle composed of protons and neutrons. However, emission often accompanies other types of radiation. When a ray is emitted during radioactive decay, it is added to the products as or . For example, when decays to , two rays of different energies are emitted. They can be added to the decay equation. Notice that the sum of the mass numbers of thorium and helium is equal to the mass number of uranium, and the sum of the atomic numbers of thorium and helium is equal to the atomic number of uranium. The decay does not contribute to mass number or atomic number changes.
Types of Radioactive Decay
|Type||Nuclear Equation||Representation||Change in Mass Number and Atomic Number|
|Alpha () decay||Mass number: –4
Atomic number: –2
|Beta () decay||Mass number: No change
Atomic number: +1
|Gamma () decay||Mass number: No change
Atomic number: No change
|Positron emission||Mass number: No change
Atomic number: –1
|Electron capture||Mass number: No change
Atomic number: –1
Uranium Decay Series
Importantly, radiocarbon dating can only be used to determine the age of once-living things. Nonliving things do not take in atmospheric carbon and thus cannot be dated in this manner, although rock strata may be dated based on the ages of dead organisms embedded in them. For dating nonliving matter, other radiometric dating methods are used. One such method is uranium-lead (U-Pb) dating. The mineral zircon incorporates uranium and thorium, but not lead, into its crystal structure. Analysis of the amount of lead in these crystals gives information that can be used to date the rocks in which they are found. U-Pb dating is precise to about 2 million years and can be used for rocks as old as 4.5 billion years. The method was initially created in order to accurately determine the age of Earth. Other radiometric dating methods, such as samarium-neodymium, potassium-argon, and rubidium-strontium, are also used for similar purposes.