3.5.4 Isotopes of Uranium

This topic explains the three isotopes of uranium — uranium-234, uranium-235, and uranium-238 — including their structure, natural abundance, and applications.

Definition

Isotopes of uranium are atoms of uranium that have the same atomic number (92 protons) but different mass numbers due to different numbers of neutrons.

Simple explanation: All uranium atoms have 92 protons. But they can have different numbers of neutrons — giving three natural isotopes.

Main Idea

Uranium has three isotopes with mass numbers 234, 235, and 238:

• Uranium-234 (²³⁴U)
• Uranium-235 (²³⁵U)
• Uranium-238 (²³⁸U)

All three have 92 protons (that's why they are all uranium), but they differ in the number of neutrons.

Core Concept

Natural abundance:

Simple explanation: Almost all natural uranium is uranium-238. Uranium-235 is a small fraction. Uranium-234 is extremely rare.

Detailed Explanation

Uranium-234 (²³⁴U)

Symbol: ²³⁴U

• 92 protons
• 142 neutrons

• It is rare.
• Natural abundance: 0.006%.
• It is part of the uranium decay chain.

Simple explanation: Uranium-234 is the rarest of the three natural uranium isotopes.

Uranium-235 (²³⁵U)

Symbol: ²³⁵U

• 92 protons
• 143 neutrons

• It is used in nuclear reactors and atomic bombs.
• Natural abundance: 0.72%.
• It is fissile — its nucleus can split when hit by a neutron, releasing energy.
• The fission of uranium-235 yields smaller nuclei, neutrons, and energy.

Energy comparison: The nuclear energy released by the fission of one kilogram of uranium-235 is equivalent to the chemical energy produced by burning more than 17,000 kg of coal.

Simple explanation: Uranium-235 is the "useful" isotope of uranium — it can produce huge amounts of energy through nuclear fission.

Uranium-238 (²³⁸U)

Symbol: ²³⁸U

• 92 protons
• 146 neutrons

• It is the most abundant isotope of uranium.
• Natural abundance: 99.27%.
• It lacks the properties necessary for nuclear reactors and atomic bombs.
• It is fertile — it can be converted into plutonium-239 in a reactor.

Simple explanation: Uranium-238 is the most common form of uranium, but it cannot be used directly for nuclear energy or weapons.

Uranium-238 Decay

When uranium-238 decays into thorium-234, it emits an alpha particle.

Nuclear equation:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

An alpha particle is a doubly positively charged helium nucleus.

Simple explanation: Uranium-238 is radioactive. It decays by emitting an alpha particle (2 protons + 2 neutrons) and becomes thorium-234.

Fission of Uranium-235

The fission of uranium-235 yields:

• Smaller nuclei
• Neutrons
• Energy

Example (simplified):

²³⁵₉₂U + ¹₀n → smaller nuclei + neutrons + energy

Simple explanation: When a neutron hits uranium-235, the nucleus splits into smaller pieces and releases a huge amount of energy.

Intermediate Level

Why Uranium-235 Is Special

Uranium-235 is fissile, meaning its nucleus can be split by a slow neutron. This releases:

• Energy (used in nuclear power plants)
• More neutrons (which can trigger further fission — a chain reaction)

Uranium-238 is not fissile — it captures neutrons but does not undergo fission easily.

Why Uranium-238 Is More Abundant

Uranium-238 has a longer half-life (about 4.5 billion years) than uranium-235 (about 700 million years). Over time, more uranium-238 has survived, making it more abundant today.

Applications of Uranium Isotopes

Energy Comparison

The book gives this striking comparison:

1 kg of uranium-235 (fission) ≈ 17,000 kg of coal (burning)

This shows how much more energy nuclear reactions release compared to chemical reactions.

Important Points

• - Uranium has three isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• - All have 92 protons (same element).
• - ²³⁴U: 0.006% abundant; rare.
• - ²³⁵U: 0.72% abundant; used in nuclear reactors and atomic bombs.
• - ²³⁸U: 99.27% abundant; lacks properties needed for reactors and bombs.
• - Uranium-238 decay: emits an alpha particle (He²⁺) and becomes thorium-234.
• - Fission of uranium-235 yields smaller nuclei, neutrons, and energy.
• - Energy comparison: 1 kg of ²³⁵U ≈ 17,000 kg of coal.

Remember

• - Uranium isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• - All have 92 protons (same element).
• - ²³⁴U: rare (0.006%).
• - ²³⁵U: used in reactors/bombs (0.72%).
• - ²³⁸U: most abundant (99.27%); not directly usable in reactors.
• - U-238 decays into Th-234 by emitting an alpha particle.
• - Fission of U-235 releases huge energy: 1 kg ≈ 17,000 kg of coal.

Common Mistakes

• 1. Thinking all uranium isotopes are used in nuclear reactors. Only ²³⁵U is directly usable; ²³⁸U is not.
• 2. Thinking uranium-238 is fissile. It is not — only ²³⁵U is fissile.
• 3. Confusing alpha decay with beta decay. U-238 undergoes alpha decay to become Th-234.
• 4. Thinking uranium-235 is the most abundant isotope. Uranium-238 is the most abundant (99.27%).
• 5. Forgetting the energy comparison. 1 kg of ²³⁵U ≈ 17,000 kg of coal — a huge difference.
• 6. Thinking uranium-234 is common. It is extremely rare (0.006%).

Quick Revision

• Uranium has three isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• All have 92 protons (same element).
• ²³⁴U: 0.006% abundant; rare.
• ²³⁵U: 0.72% abundant; used in nuclear reactors and atomic bombs.
• ²³⁸U: 99.27% abundant; lacks properties needed for reactors/bombs.
• Uranium-238 decay: emits alpha particle (He²⁺), becomes thorium-234.
• Fission of uranium-235: yields smaller nuclei, neutrons, and energy.
• Energy comparison: 1 kg of ²³⁵U ≈ burning of 17,000 kg of coal.

60-Second Revision

• Uranium isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• All have 92 protons.
• ²³⁴U: 0.006% — rare.
• ²³⁵U: 0.72% — reactors/bombs.
• ²³⁸U: 99.27% — most abundant.
• U-238 decays into Th-234 (alpha emission).
• Fission of U-235 releases huge energy.
• Warning: only ²³⁵U is fissile; ²³⁸U is not.