Radioactive decay is a natural process where an unstable atomic nucleus releases energy and particles to transform into a more stable form, altering the element’s identity (transmutation). The specific path from Thorium-230 to Radium-226 provides a clear illustration of this fundamental process. This single-step decay is a significant part of the long-running radioactive series occurring naturally within the Earth’s crust.
Understanding Alpha Decay
The process by which Thorium-230 converts to Radium-226 is alpha decay, where the parent atom ejects a large, energetic particle from its nucleus. This emitted alpha particle is identical to a Helium-4 nucleus, consisting of two protons and two neutrons. It has an atomic mass of four units and a positive two electrical charge.
The emission of this particle fundamentally changes the atom’s identity. Since the nucleus loses two protons, the atomic number decreases by two, shifting its position on the periodic table. The loss of two protons and two neutrons causes the atomic mass number to decrease by four. This nuclear rearrangement releases a specific amount of energy as the nucleus moves to a lower-energy state.
Alpha decay occurs in heavy, unstable nuclei that have too many protons to be held together stably by the strong nuclear force. Emitting the alpha particle is the nucleus’s method of reducing its size and becoming more stable. Although alpha particles are massive and energetic, they interact strongly with matter and have a very short range, easily stopped by a sheet of paper.
The Thorium-230 to Radium-226 Transformation
The transformation begins with Thorium-230 (\(\text{}^{230}_{90}\text{Th}\)), an isotope with 90 protons and 140 neutrons. This unstable nucleus spontaneously ejects an alpha particle, leading directly to the formation of Radium-226 (\(\text{}^{226}_{88}\text{Ra}\)). The nuclear equation representing this transmutation is \(\text{}^{230}_{90}\text{Th} \rightarrow \text{}^{226}_{88}\text{Ra} + \text{}^{4}_{2}\text{He}\).
The resulting Radium-226 nucleus contains 88 protons and 138 neutrons. This single-step decay event releases a substantial amount of energy, approximately \(4.7\) million electron volts (MeV) per decay. This energy is primarily carried away by the kinetic movement of the alpha particle and the recoiling Radium-226 nucleus.
The rate of this transformation is defined by the half-life of Thorium-230, which is approximately 75,400 years. This half-life means that for any given quantity of Thorium-230, half of it will have decayed into Radium-226 after this period. The long half-life indicates that Thorium-230 is an intermediate step in the radioactive decay series, continuously feeding the Radium-226 population in the environment.
Context and Importance of These Isotopes
Thorium-230 and Radium-226 are naturally occurring radioisotopes belonging to the long decay chain of Uranium-238, the most abundant uranium isotope found in nature. Thorium-230 is generated from the alpha decay of Uranium-234, which is a product of the U-238 chain. Radium-226 continues the series by decaying into the radioactive gas Radon-222.
The distinct chemical properties of these isotopes make them invaluable tools in geoscience. Thorium is largely insoluble in natural waters, unlike its parent element Uranium. This chemical difference is the basis for uranium-thorium dating, used to determine the age of calcium carbonate materials like corals and cave formations (speleothems). By measuring the ratio of the daughter isotope Thorium-230 to its parent Uranium-234, scientists can date materials up to about 500,000 years old.
Radium-226, with a half-life of about 1,600 years, has a complex history of use and concern. It was historically employed in self-luminous paints for watch dials and aircraft instruments, leading to severe health issues for workers. It was also used in sealed sources for cancer therapy, though safer alternatives have replaced this approach. Today, Radium-226 remains an environmental concern because its presence in soil and groundwater contributes to natural background radiation and poses an internal hazard if ingested.