Uranium

Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. Uranium radioactively decays, which means it slowly changes over time and releases energy. This property helps scientists learn about the age of the Earth.

Uranium has many important uses today. It is used in nuclear power plants to produce electricity and in nuclear weapons. This is because one of its types, called uranium-235, can split easily and release a lot of energy. However, uranium needs special treatment to increase the amount of uranium-235 before it can be used.

Uranium was first found in 1789 by a scientist named Martin Heinrich Klaproth, who named it after the planet Uranus. Later, scientists discovered its radioactive nature and began using it in powerful nuclear reactions. This led to the creation of many nuclear weapons during the Cold War between the United States and the Soviet Union. Today, efforts are made to safely handle and reuse materials from these old weapons.

Characteristics

Uranium is a shiny, white metal that gives off a little bit of radiation. It is strong enough to scratch glass and is similar in hardness to metals like titanium. Uranium is soft and can be shaped, and it is not a great conductor of electricity. It is very heavy, heavier than lead but not as heavy as gold.

Uranium can react with many elements and compounds, especially when it gets hotter. Special types of acids can dissolve uranium. Tiny pieces of uranium can even react with cold water. In 1938, scientists discovered that uranium can split into smaller pieces when hit with tiny particles called neutrons. This splitting can create a lot of energy and more neutrons. If too many neutrons hit more uranium, it can cause a big chain reaction, like in a nuclear reactor or bomb.

Applications

Military

The main use of uranium in the military is in special ammunition that can break through very strong armor. This ammunition is made from a special type of uranium mixed with a tiny amount of other metals. It is very heavy and hard, which helps it destroy enemy tanks and vehicles. This same special uranium is also used to make armor stronger.

Uranium has also been used to make nuclear weapons. During World War II and the Cold War, a special kind of uranium was used to create powerful explosions.

Civilian

Mostly, uranium is used to make electricity in nuclear power plants. A small amount of uranium can produce a huge amount of energy.

Before people knew about radioactivity, uranium was used to color glass and pottery. It was also used in old photography, lighting, and even to help make teeth look better. Scientists also use uranium to figure out how old very old rocks are.

History

Pre-discovery use

People have used a natural form of uranium, called pitchblende, for a very long time. As far back as 79 AD, during the time of the Roman Empire, pitchblende was used to give a yellow color to ceramic glazes. Later, during the Middle Ages, it was taken from silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic), and used to color glass.

Discovery

The element uranium was discovered by a German chemist named Martin Heinrich Klaproth in 1789. He was working in Berlin and found a yellow substance in pitchblende. He thought it was a new element and named it "Uranium" after the planet Uranus, which had been discovered eight years earlier.

In 1896, a scientist named Henri Becquerel discovered that uranium could emit invisible rays that could affect photographic plates. This was the first discovery of radioactivity.

Fission research

In 1934, a team led by Enrico Fermi found that when uranium is bombarded with neutrons, it can break apart in a process called nuclear fission. This discovery led to the development of nuclear power and nuclear weapons.

Nuclear weaponry

During World War II, the United States developed two types of atomic bombs. One used uranium, and the other used plutonium made from uranium. The uranium bomb was used over Hiroshima, Japan, in 1945.

Reactors

The first reactor designed to run continuously was built in Oak Ridge, Tennessee. Another reactor in Idaho was the first to produce electricity. The world’s first commercial nuclear power plant began operating in the Soviet Union in 1954.

Prehistoric naturally occurring fission

In 1972, scientists found evidence of natural nuclear reactors that operated over a billion years ago in Gabon, Africa. These reactors worked because there was more uranium-235 available at that time.

Contamination and the Cold War legacy

Above-ground nuclear tests in the 1950s and 1960s spread radioactive materials around the world. Uranium miners have faced health risks, including cancer. During the Cold War, large amounts of uranium were stored, and there were concerns about security and safety. Efforts have been made to improve safety and secure nuclear materials.

Occurrence

Uranium is a natural element that can be found in small amounts in rocks, soil, and water everywhere on Earth. It is the heaviest element that is commonly found in nature and is usually combined with other elements. Uranium is about the 48th most common element in the Earth's crust.

Uranium is not very plentiful, but it is more common than metals like silver, tin, or mercury. It is found in many minerals, and some places have lots of it, like certain types of rock and sand. Scientists think that most of Earth's heat comes from the slow breaking down of uranium and other elements deep inside the planet.

Compounds

Oxidation states and oxides

See also: Uranium oxide

Triuranium octoxide and uranium dioxide are the two most common uranium oxides.

Uranium can exist in different forms called oxidation states. The two most important are uranium(IV) and uranium(VI). These form two oxides: uranium dioxide (UO₂) and uranium trioxide (UO₃). Other oxides like uranium monoxide (UO), diuranium pentoxide (U₂O₅), and uranium peroxide (UO₄·2H₂O) also exist.

The most common forms of uranium oxide are triuranium octoxide (U₃O₈) and UO₂. Both are solids that don’t dissolve easily in water and stay stable under many conditions. Triuranium octoxide is the most stable form of uranium and is often found in nature. Uranium dioxide is used as fuel in nuclear reactors. Over time, UO₂ changes into U₃O₈. Because they are stable, uranium oxides are good for storing or disposing of uranium.

Aqueous chemistry

Many uranium salts can dissolve in water and are studied in water solutions. Common forms include U³⁺ (brown-red), U⁴⁺ (green), UO⁺₂ (unstable), and UO²⁺₂ (yellow). Some solid compounds like UO and US exist but don’t form simple ions in water. U³⁺ can release hydrogen from water and is very unstable. The UO²⁺₂ ion forms compounds like uranyl carbonate, uranyl chloride, and uranyl sulfate. It also forms complexes with organic chelating agents, such as uranyl acetate.

Unlike uranyl salts, uranates (salts with a uranium-oxide anion) usually don’t dissolve in water.

Carbonates

When carbonate is added to uranium(VI) solutions, it changes how uranium behaves. Most carbonates don’t dissolve in water, but uranium carbonates often do. This is because uranium can bind with carbonates to form soluble complexes.

Effects of pH

The way uranium behaves changes with the pH of the solution. When pH increases, uranium turns into a hydrated uranium oxide hydroxide and then into an anionic hydroxide complex. Adding carbonate changes uranium into carbonate complexes, increasing its solubility between pH 6 and 8. This affects the long-term stability of used uranium dioxide nuclear fuels.

Hydrides, carbides and nitrides

Uranium metal reacts with hydrogen when heated to form uranium hydride. At higher temperatures, the hydrogen can be removed. This makes uranium hydrides useful for creating reactive uranium powder and other compounds like uranium carbide, nitride, and halide. Uranium hydrides have two forms: an α form made at low temperatures and a β form made above 250°C.

Uranium carbides and uranium nitrides are inert semimetallic compounds that don’t dissolve in acids, react with water, and can catch fire in air to form U₃O₈. Examples of uranium carbides include uranium monocarbide (UC), uranium dicarbide (UC₂), and diuranium tricarbide (U₂C₃). Uranium nitrides include uranium mononitride (UN), uranium dinitride (UN₂), and diuranium trinitride (U₂N₃).

Halides

All uranium fluorides are made using uranium tetrafluoride (UF₄). UF₄ is made by adding hydrofluoric acid to uranium dioxide. Reducing UF₄ with hydrogen makes uranium trifluoride (UF₃). Under certain conditions, UF₄ reacts with uranium hexafluoride (UF₆) to form intermediate fluorides.

At room temperature, UF₆ easily turns into a vapor, which is useful for separating uranium-235 from uranium-238. UF₆ is made by reacting UO₂ with hydrofluoric acid and then UF₄ with fluorine gas.

Uranium hexafluorides can form hexafluorouranates, which bond with alkali metals, some transition metals, and other non-metal compounds.

Uranium tetrachloride (UCl₄) is made by combining chlorine with uranium metal or uranium hydride. Reducing UCl₄ with hydrogen makes uranium trichloride (UCl₃). All uranium chlorides react with water and air.

Bromides and iodides of uranium are made by reacting bromine and iodine with uranium or by adding uranium hydride to their acids. Examples include UBr₃, UBr₄, UI₃, and UI₄. Uranium oxyhalides, such as UO₂F₂, UOCl₂, UO₂Cl₂, and UO₂Br₂, dissolve in water. Their stability decreases as the atomic weight of the halide increases.

Isotopes

Main article: Isotopes of uranium

Uranium, like all elements with more than 82 protons, does not have stable isotopes. All of its isotopes are radioactive. However, two isotopes, ²³⁸U and ²³⁵U, have very long lifetimes and can still be found in nature.

Natural uranium consists mainly of three isotopes: uranium-238 (about 99.28%), uranium-235 (about 0.71%), and a very small amount of uranium-234 (about 0.0054%). There are also a few other rare isotopes of uranium.

Uranium-238 is the most stable isotope, with a very long lifetime. It decays into thorium-234 and eventually into lead-206. Uranium-235 has a shorter lifetime than uranium-238 but is still very long. It is important for nuclear reactors and weapons because it can be split by certain particles, releasing energy. Uranium-234 is part of the same decay series as uranium-238 and also decays into lead-206.

Other isotopes of uranium, like uranium-236 and uranium-233, are produced in nuclear reactors and have shorter lifetimes. Some of these isotopes can also be used as nuclear fuel.

Human exposure

People can come into contact with uranium or its radioactive daughters, like radon, by breathing in dust or eating food and drinking water that contain it. Usually, there is only a tiny amount of uranium in the air, but people may be exposed to more if they work in factories that process phosphate fertilizers with uranium, live near places that make or test nuclear weapons, or are near battlefields where special weapons have been used. Living near coal-fired power plants, uranium mines, or uranium processing facilities can also increase exposure.

The health effects of uranium come mainly from its toxicity, not from radiation. Special groups, like the Occupational Safety and Health Administration, have set limits on how much uranium workers can be around in a day. Most uranium that is eaten passes through the body, but some can stay in the body for many years, especially in bone tissue. Uranium does not go through the skin, and the small particles it gives off cannot pass through skin either.

Effects and precautions

Uranium can affect the kidney, brain, liver, heart, and other parts of the body because it is a toxic metal. While it gives off a type of radiation that cannot go through skin, inhaling uranium can be harmful. Special care is taken when handling uranium to protect people from breathing it in or swallowing it.