Uranium Mass Number

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Uranium (chemical symbol U) is a naturally occurring radioactive element. When refined, uranium is a silvery-white metal. Uranium has three primary naturally occurring : U-238, U-235 and U-234.

Uranium is weakly radioactive and contributes to low levels of natural in the environment. Uranium is used in nuclear power generation. Specifically, U-235 can be concentrated in a process called “enrichment,” making it 'fissile' and suitable for use in nuclear reactors or weapons.

Type of Radiation Emitted:

(from radioactive decay products)
Uranium-238: 4.47 billion years
Uranium-235: 700 million years
Uranium-234: 244,000 years

Name: Uranium Symbol: U Atomic Number: 92 Atomic Mass: 238.0289 amu Melting Point: 1132.0 °C (1405.15 K, 2069.6 °F) Boiling Point: 3818.0 °C (4091.15 K, 6904.4 °F) Number of Protons/Electrons: 92 Number of Neutrons: 146 Classification: Rare Earth Crystal Structure: Orthorhombic Density @ 293 K: 18.95 g/cm 3 Color: silverish Atomic Structure.

Uranium Mass Number

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Mass numbers of typical isotopes of Uranium are 238, 235. The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Neutron number plus atomic number equals atomic mass number: N+Z=A. Mass number: number of protons + number of neutrons Natural isotopes of uranium are with the mass numbers: 234U: 234 nucleons 235U: 235 nucleons 238U: 238 nucleons Mass number for uranium?

Uranium in the Environment

Uranium is present naturally in virtually all soil, rock and water. Rocks break down to form soil. Soil can be moved by water and blown by wind, which moves uranium into streams, lakes and surface water. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium-234 is less than one percent of all forms of natural uranium, but is much more radioactive. It gives off almost half of the radioactivity from all forms of uranium found in the environment.


The U.S. mining industry can retrieve uranium in two ways. The first is to mine rock that contains uranium. The second is to use strong chemicals to dissolve uranium from underground rocks into ground water, and then pump the water to the surface. The waste from these processes is more radioactive than the natural rock because the natural radioactive material in the earth is now exposed and concentrated. This waste can contaminate water, soil and air if it is not disposed of properly. Uranium eventually decays to radium. Radium decays to release a radioactive gas called radon. Radon in underground uranium mines is a greater radiation hazard to miners than uranium. Without precautions (i.e. ventilation) radon can collect in the mine shafts where it is inhaled by miners. Learn more about uranium mines and mills.

Uranium Sources

A person can be exposed to uranium by inhaling dust in air, or ingesting water and food. The general population is exposed to trace levels of uranium primarily through food and water. Learn about background radiation.

People who live near federal government facilities that made or tested nuclear weapons, or facilities that mine or process uranium ore or enrich uranium for reactor fuel, may have increased exposure to uranium. Uranium that is depleted (U-235) is used in industrial settings (i.e. counterweights).

School science labs may keep small quantities of uranium of varying enrichment levels to demonstrate radioactive properties. These sources have low levels of radioactivity and are not harmful to people when handled properly.

Ingestion of uranium is a hazard because of its chemical properties.

Uranium and Health

Uranium Protons Neutrons Electrons

Uranium Mass Number

Uranium decays by alpha particles. External exposure to uranium is therefore not as dangerous as exposure to other radioactive elements because the skin will block the alpha particles. Ingestion of high concentrations of uranium can cause health effects, such as cancer of the bone or liver. Inhaling large concentrations of uranium can cause lung cancer from the exposure to alpha particles.

Although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. If the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.

At the point where the chain reaction can become self-sustaining, this is referred to as critical mass.

In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes

Uranium Element Mass Number

The amount of a fissionable material's critical mass depends on several factors; the shape of the material, its composition and density, and the level of purity.

A sphere has the minimum possible surface area for a given mass, and hence minimizes the leakage of neutrons. By surrounding the fissionable material with a suitable neutron 'reflector', the loss of neutrons can reduced and the critical mass can be reduced.

By using a neutron reflector, only about 11 pounds (5 kilograms) of nearly pure or weapon's grade plutonium 239 or about 33 pounds (15 kilograms) uranium 235 is needed to achieve critical mass.