The Unbelievable Element Uranium

Introduction to Uranium

The element uranium is a dull gray to silvery actinide metal. Uranium is primarily known for its high atomic weight, as well as its radioactive properties. Because of its radioactivity, uranium has many important uses today, including in electricity generation and military weapons development.

10 Fun Facts about Uranium

Uranium has the highest atomic weight (238.03) and atomic number (92) of any other naturally occurring element on Earth.
The name “Uranium” comes from the planet Uranus, which itself is named after the Greek god of the sky. Martin Heinrich Klaproth, the scientist credited with discovering uranium (1789), chose the name in celebration of Uranus’ then-recent discovery (1781).
Depleted uranium (free of radioactive isotopes) has some use in dense armor-piercing ammunition.
Naturally occurring uranium-fueled fission reactors have been discovered to exist in West Africa, dated to be 2 billion years old. Though inactive today, scientists theorize that a species of bacteria could have concentrated uranium-235 because they use uranium as a metabolic catalyst and preferentially select uranium-235.
Until the 20th century, humans primarily used uranium in ceramics and glass to add a yellow-green color. This practice was particularly widespread in modern-day Germany and Czechia during the middle ages using pitchblende.
Although known for radioactivity, uranium poses more danger from its toxicity, which can damage organs and cause neurological disorders.
The decay of uranium, as well as thorium and potassium-40, serves as the driving factor behind the heat of the Earth’s mantle. This heat consequently plays an incredibly important role in plate tectonics.
In 1896, French physicist Henri Becquerel discovered the phenomenon of radioactivity through work with uranium.
Uranium has a remarkable energetic density from nuclear fission. One kilogram of uranium ore produces 1404 Megajoules of energy, compared to 93 MJ for 1kg of coal. Intriguingly, one kilogram of pure uranium-235 produces 5,184,000 MJ.
Fine uranium oxide powder is known to spontaneously combust at temperatures of 300°F, posing a serious risk at nuclear power plants, which frequently involve high temperatures.

Uranium in the Periodic Table

Uranium has the atomic symbol U with an atomic number of 92. It locates in the f-block of the periodic table, with protactinium to its left and neptunium to its right. Uranium occupies the fourth element in the Actinide series of the periodic table. Uranium has an electron configuration of [Rn]5f³6d¹7s². Additionally, the element has an electronegativity of 1.38 on the Pauling scale.

Uranium’s Application in Today’s World

What is uranium element used for?

Uranium has many diverse uses in research, particularly for scientists interested in radioactivity and nuclear fission or radioactive dating. However, warfare and energy provide the most famous and impactful uses of uranium.

Uranium in Warfare

After uranium’s discovery in 1789, the element interested chemists very little for more than a century. However, in 1934, the team working under Italian physicist Enrico Fermi discovered that uranium could emit beta rays. Just a few years later, in 1939, German physicists Otto Hahn and Fritz Strassman discovered the properties of uranium in performing nuclear fission. These findings intrigued many physicists, as they implied that uranium fission could consequently release vast amounts of energy. At the end of the 1930s, many envisioned, with both horror and excitement, the possibility of nuclear fission being exploited to create bombs with unparalleled destruction. After the Second World War began, this possibility then became reality with the birth of the Manhattan Project in 1942.

trinity nuclear explosion, a demonstration of the power of uranium element

Between laboratories in Los Alamos, NM, Oak Ridge, TN, and Chicago, an international team of scientists began work on the first nuclear weapons. This team included Enrico Fermi, J. Robert Oppenheimer, Leo Szilard, Glenn Seaborg, and dozens of others. After years of research and testing, the first nuclear bomb test then occurred in rural New Mexico on July 16, 1945. Less than a month later, a uranium nuclear bomb, code-named “Little Boy”, would be used in warfare for the first and only time on August 6th in the Japanese city of Hiroshima, killing over 200,000 civilians. After the bombing of Nagasaki on August 9, with the plutonium-based “Fat Man” bomb, beginning the age of nuclear warfare.

With the end of the Second World War, the Cold War subsequently pushed many countries on both sides of the conflict to develop their nuclear weapons programs. Even after the Cold War, most countries such as the United States, Russia, and China retained their nuclear weapon stockpiles. Thankfully, no wars since the Second World War have involved nuclear bombs.

Uranium in Nuclear Energy

Aside from warfare, uranium also has an important use in generating electricity through nuclear fission. Specifically, when uranium nuclei split, they release an incredible amount of energy which rapidly converts to heat. When fission occurs in water, the water heats and phase-changes to steam, which can then activate turbines, thus generating electricity.

However, nuclear scientists must heavily process uranium ore to produce usable nuclear fuel. The nuclear industry most often uses yellowcake uranium ore (U₃O₈) as a source of uranium. Of the uranium atoms in yellowcake, only 0.7% of them are uranium-235, the isotope necessary for reliable nuclear fission. To enrich the uranium to reach the minimum proportion of 3% of ²³⁵U, the uranium oxide in yellowcake reacts to form UF₆ (see Uranium Compounds and Reactions). When converted to the gas phase, UF₆ allows for easy separation of ²³⁵U through gaseous diffusion.

Nuclear energy has existed since uranium first interested scientists and militaries for use in warfare. However, nuclear energy has gained considerable interest in recent years as a way of combatting climate change. Unlike coal or natural gas, uranium- or plutonium-fueled nuclear reactors do not directly emit CO₂ to produce electricity. Nevertheless, issues such as safety, uranium ore availability, efficiency, and cost have caused many to doubt whether nuclear energy is the best energy source to combat climate change.

Where is uranium element found?

Mining companies generally extract uranium in the form of triuranium octoxide (U₃O₈), present between 0.25%-23% in mineral ores. To concentrate the uranium oxide, ores become processed and leached with strong acid or base, later generating yellowcake uranium (>75% U₃O₈).

Most uranium ores are located in central Eurasia, North America, and Australia, with Kazakhstan, Canada, and Australia producing nearly half of the world’s yellowcake uranium. Significant uranium mining operations also exist in Russia, Niger, Russia, and the United States.

When and How was the Element Uranium Discovered?

The element uranium first became known to humanity in 1789 by the German chemist Martin Heinrich Klaproth. Specifically, Klaproth produced a greenish-yellow substance in the shape of hexagonal prisms from dissolving a sample of pitchblende in nitric acid. Though Klaproth had then later claimed to have isolated the new element, many scientists correctly guessed that he had only discovered an oxide compound. Not until more than half a century later, in 1841, would someone (French chemist Eugéne-Melchior Péligot) successfully isolate elemental uranium.

Uranium Chemistry – Compounds, Reactions, Isotopes, Oxidation States

Uranium Compounds and Reactions

Many different compounds involve uranium. The most notable classes of uranium compounds include:

Oxides: U₃O₈, UO₂, UO₃, UO, U₂O₅
Carbides: UC, U₂C₃, UC₂
Nitrides: UN, U₂N₃, UN₂
Halides: UF₆, UF₄, UCl₄, UCl₃, UBr₃, UBr₄, UI₃, UI₄

To convert between these different compounds, uranium performs a wide variety of reactions.

In nuclear fuel production, scientists must perform a series of reactions to convert uranium oxide into uranium hexafluoride:

U₃O₈ + 6HNO₃ → 3UO₂(NO₃)₂ + 2H₂O + 2H⁺

4UO₂(NO₃)₂ + 4NH₃ + 4H⁺ + 3O₂ → 2(NH₄)₂U₂O₇ + 4NO₃^–

(NH₄)₂U₂O₇ + 2H₂ → 2UO₂ + 2NH₃ + 3H₂O

UO₂ + 4HF → UF₄ + 2H₂O

UF₄ + F₂ → UF₆

Additionally, uranium can convert to its elemental form, where its oxidation state equals zero. Elemental uranium can then convert to all oxides, carbides, nitrides, and halides with other elemental forms at high temperatures:

4U + 5O₂ → 2U₂O₅

C + U → UC

N₂ + U → UN₂

U + nX₂ → UX_2n (X = F, Cl, Br, I) (n = 2-3)

Isolation of Elemental Uranium

Relative to most other metals, uranium has a reputation for being remarkably difficult to isolate. In 1841, Eugéne-Melchior Péligot successfully isolated uranium by heating uranium tetrachloride with potassium.

Instead, nuclear scientists use uranium for nuclear energy and military weaponry in the form of pellets of uranium dioxide (UO₂). After enriching through gaseous diffusion of uranium hexafluoride (UF₆), scientists then reduce the uranium to UO₂.

Uranium Isotopes

Uranium primarily comes in three isotopes:

Uranium-238 (²³⁸U)
- Natural Abundance: 99.28%
- Radioactive Half-life: 4.47 x 10⁹ years
- Decay Chain: Uranium/Radium Series
- Notes: The isotope most commonly found on Earth. Though not used in nuclear fission, ²³⁸U can undergo neutron absorption, which converts to plutonium-239 through beta emission. ²³⁹Pu, conversely, can undergo nuclear fission.
Uranium-235 (²³⁵U)
- Natural Abundance: 0.71%
- Radioactive Half-life: 7.13 x 10⁸ years
- Decay Chain: Actinium Series
- Notes: The isotope generally used in nuclear weapons and nuclear energy. Unlike other isotopes, ²³⁵U is fissile, meaning that can split into fragments through nuclear fission. These fragments then collide with subsequent ²³⁵U nuclei, beginning the nuclear chain reaction.
Uranium-234 (²³⁴U)
- Natural Abundance: 0.0054%
- Radioactive Half-life: 2.48 x 10⁵ years
- Decay Chain: Uranium/Radium Series

Uranium Oxidation States

Uranium’s usual oxidation states include:

U(-I): U^–, [U(CO)₈]^–
U(I): U⁺, U(C₆H₆)⁺
U(II): U²⁺, UO, UO₂F₂
U(III): U³⁺, UN, UH₃
U(IV): U⁴⁺, UO₂, UF₄
U(V): U⁵⁺, U₂O₅, UF₅
U(VI): U⁶⁺, UN₂, UF₆

Properties of Uranium Element

Atomic Symbol: U
Melting point: 1503K; 1132°C; 2070°F
Boiling point: 4404K; 4131°C; 7468°F
Density: 19.1 g/ml
Atomic weight: 238.0289
Atomic number: 92
Electronegativity: 1.38
Molar heat capacity: 27.67 J/(mol*K)
Classification: Actinide
Natural abundance in the Earth’s crust: 2.7 ppm
Electron shell configuration: [Rn]5f³6d¹7s²
Stable Isotopes: 234, 235, 238
Found naturally in the minerals: uraninite (pitchblende), finchite, carnotite, coffinite
Toxicity: Toxic

Where Can I Buy Uranium Element?

Small quantities of pure uranium-238 can be bought from online vendors specializing in samples for educational purposes. Chemical suppliers like Sigma Aldrich also sell certain uranium salts.