Question

Identify two of the most abundant radioactive elements that exist on Earth. Explain why they are still present. (You may wish to consult a website such as that of the University of Sheffield and WebElements Ltd, UK, webelements.com.)

Short answer

Uranium and thorium are abundant due to their long half-lives, persisting over billions of years.

Step-by-step solution

Introduction to Radioactive Elements

Radioactive elements are atoms that decay over time, releasing particles and energy. Some of these elements are still present on Earth due to their long half-lives, which allow them to last for billions of years. Two of the most common radioactive elements are uranium and thorium.

Identifying Uranium

Uranium is a heavy radioactive element commonly found in rocks, soil, and water. It occurs naturally in several isotopes, with uranium-238 being the most abundant. With a half-life of about 4.5 billion years, uranium-238 remains present in significant amounts.

Identifying Thorium

Thorium is another naturally occurring radioactive element. It is found in small amounts in most rocks and soils. Thorium-232 is the most common isotope, and it has a half-life of about 14 billion years, making it abundant on Earth.

Explanation of Longevity

The long half-lives of uranium-238 and thorium-232 mean that even after billions of years, a substantial quantity of these elements remains. This extended radioactivity is why these elements are prevalent on Earth today.

Key concepts

Uranium

Uranium is one of the heaviest and most well-known radioactive elements. As a naturally occurring element, you'll find it in the Earth's crust, rocks, soil, and even in water. It exists in various isotopes, with uranium-238 making up the largest share. This isotope is notably stable compared to others.
The importance of uranium comes from its use as fuel in nuclear power plants. Its ability to undergo fission means it can split and release a large amount of energy. This makes it a valuable resource for generating electricity. The presence of uranium today is largely due to its immense half-life. It takes around 4.5 billion years for half of any given sample of uranium-238 to decay. Given this time span, much of the uranium present during Earth's formation remains today, although it continuously radiates energy.

Thorium

Thorium is another naturally occurring radioactive element, slightly lighter than uranium and often found in a variety of minerals. Found in small amounts within soil and rocks, its most plentiful isotope is thorium-232.
Unlike uranium, thorium is more abundant in the Earth's crust, presenting it as a potential energy source for the future. While not as commonly used today, the interest in thorium is resurging due to its potential efficiency in nuclear reactors. Thorium-232 enjoys an incredibly long half-life of about 14 billion years. This means that it takes that long for half of a sample to decay. Such an extended half-life allows thorium to remain plentiful on Earth, maintaining its stability across millennia.

Half-life

The concept of half-life is critical to understanding why elements like uranium and thorium endure over time. A half-life is the time required for half the atoms in a radioactive element to decay. This period varies with different radioactive substances, explaining their varied longevity.
For example, uranium-238 has a half-life of about 4.5 billion years. This long half-life means its decay is very gradual, allowing substantial quantities to still exist since Earth's formation. Thorium-232's half-life is even longer, approximately 14 billion years, making it one of the most stable radioactive elements. These extended half-lives are the reason both uranium and thorium are still prevalent. As they decay slowly, these elements have survived across geological time scales, contributing to their persistence in the Earth's crust.