Difference Between U 235 and U 238

Main Difference – U 235 vs U 238

Radioactive elements are compounds that break down over time by releasing energy and turning into different elements. This happens because these elements are not stable. In order to become stable, they release energy through radioactive decay. Almost all elements are found in several forms known as isotopes. Some of these isotopes are very stable in nature. But other isotopes are not stable and they undergo radioactive decay. These isotopes are called radioactive isotopes. However, even stable isotopes may undergo radioactive decay, but it cannot be observed since this process takes a very long time. Uranium is a chemical element well-known for its radioactive decay. U-235 and U-238 are two radioactive isotopes of Uranium. The main difference between U-235 and U-238 is that the number of neutrons present in the U-235 nucleus is 143 whereas the number of protons present in the U-238 nucleus is 146.

Key Areas Covered

1. What is U 235
      – Definition, Properties, and Uses
2. What is U 238
      – Definition, Properties, and Uses
3. What are the Similarities Between U 235 and U 238
      – Outline of Common Features
4. What is the Difference Between U 235 and U 238
      – Comparison of Key Differences

Key Terms: Half-Life, Isotope, Neutrons, Protons, Radioactive Decay, Uranium

What is U 235

U-235 is an isotope of the chemical element Uranium that is composed of 92 protons and 143 neutrons in its nucleus. The chemical symbol for Uranium is given as ²³⁵₉₂U. The natural abundance of U-235 is about 0.72%. The mass of this isotope is about 235.043 amu.

The half-life of a U-235 isotope is found to be around 700 million years. In other words, it takes about 700 million years to half its mass through radioactive decay. The mode of decay that can be seen in U-235 is alpha decay. This means U-235 releases an alpha particle when it undergoes radioactive decay.

Figure 1: The chain reaction of the fission of U-235 can be initiated with the bombardment of a high-speed neutron.

U-235 is capable of sustaining a chain reaction of nuclear fission. Therefore U-235 is fissile. A natural fission chain will end with Thorium-231, which is a stable element. The fission of one U-235 atom releases 202.5 MeV. The major uses of U-235 include the applications in nuclear weapons and nuclear power plants.

What is U 238

U-238 is an isotope of Uranium, which is composed of 92 protons and 146 neutrons in its nucleus. It is the most abundant isotope of Uranium element. The abundance of U-238 is about 99%. It is non-fissile, which means, U-238 does not undergo any chain reaction of nuclear fission.

However, they can be made to become fissile by bombardment of a high-speed neutron. Therefore, it is called a fertile material. But even with this bombardment, the probability of becoming fissile is very low. When the nucleus catches a neutron, it forms unstable U-239 isotope. This U-239 isotope is fissile and starts a chain reaction of radioactive decay.

Figure 2: Uranium-238

The half-life of U-238 is about 4.4 billion years. The molar mass of this isotope is about 238.05 amu. This isotope also tends to alpha decay. The end product of this decay is Thorium-234.

Modern nuclear weapons use U-238 as a tamper material. It covers the core containing fissile material. It is helpful in reflecting neutrons that are released and increases the efficiency of the weapon.

Similarities Between U 235 and U 238

U 235 and U 238 are isotopes of the same chemical element; Uranium.
Both are radioactive isotopes.
Both isotopes are composed of 92 protons in their nucleus.
Both are used in the production of nuclear weapons.
Both types undergo alpha decay.

Definition

U 235: U-235 is an isotope of Uranium and is composed of 92 protons and 143 neutrons in its nucleus.

U 238: U-238 is an isotope of Uranium and is composed of 92 protons and 146 neutrons in its nucleus.

Neutrons

U 235: The number of neutrons present in the U-235 nucleus is 143.

U 238: The number of neutrons present in U-238 nucleus is 146.

Half-life

U 235: The half-life of U-235 is about 703 million years.

U 238: The half-life of U-238 is about 4.4 billion years.

Abundance

U 235: The natural abundance of U-235 is about 0.72%.

U 238: The natural abundance of U-238 is about 99%.

Chain Reactions

U 235: U-235 is capable of sustaining nuclear fission chain reactions.

U 238: U-238 alone is not capable of sustaining nuclear fission chain reactions.

Atomic Mass

U 235: The mass of the U-235 atom is about 235.043 amu.

U 238: The mass of the U-238 atom is about 238.05 amu.

Conclusion

Uranium is a well-known radioactive material. It has several isotopes, and all these isotopes are radioactive in nature. U-235 and U-238 are such isotopes of Uranium. The main difference between U-235 and U-238 is that the number of neutrons present in the U-235 nucleus is 143 whereas the number of protons present in the U-238 nucleus is 146.

References:

1. “Uranium-238.” Wikipedia, Wikimedia Foundation, 24 Aug. 2017, Available here. Accessed 29 Aug. 2017.
2. “Uranium 238 and 235.” Radioactivity: Uranium 238 and 235, Available here. Accessed 29 Aug. 2017.

Image Courtesy:

1. “Kernspaltung” By Stefan-Xp – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Uranium238” By Greenhorn1 – Own work (Public Domain) via Commons Wikimedia