What is the mass number of uranium

A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (which comprises 0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons and is fertile, meaning that it becomes fissile after absorbing one neutron. Natural uranium also includes a minute amount of uranium-234.

Page Last Reviewed/Updated Tuesday, March 09, 2021

Uranium

• Was used for dying pottery, when in natural form
• The atomic bomb that was used at Hiroshima in World War 2, was uranium based
• Can be found in low levels in all rocks

• Ammunition
• High density penetrators (military)
• Ballast for re-entry aircrafts
• Gyroscopic compasses
• Fuel commercial nuclear power plants

In 1969, the Commission recommended Ar(U) = 238.029(1) for the atomic weight of U based on mass-spectrometric determinations and a careful analysis of the variability of x(235U) in nature. In 1979, the Commission took note of later studies of the variations of the 235U abundance in normal sources, which justified a more precise value for the standard atomic weight, thus leading to Ar(U) = 238.0289(1). The atomic weight and uncertainty of uranium were changed to 238.028 91(3) in 1999 on the basis of new calibrated mass-spectrometric measurements.

That value applies to uranium as found in normal terrestrial sources, except as discovered in one locality in south-west Africa (Gabon at Oklo), hence the annotation "g". Uranium is used in the nuclear fuel cycle either enriched or depleted in 235U, hence the annotation "m".

All uranium isotopes are α-emitters. Isotopes 235U and 238U are primordial with the 235U abundance declining very gradually in geological time because of its faster decay. 234U, itself a decay product of 238U, is in equilibrium established by the ratio of the half-lives. 235U decays by a branched series ending with 207Pb, 238U (and 234U) by a similar series ending in 206Pb. The 238U–206Pb and the 235U–207Pb decay systems are of fundamental importance in geochronology.

Uranium (U) is a metallic, silver-gray element that is a member of the actinide series. It is the principle fuel for nuclear reactors, but it also used in the manufacture of nuclear weapons.

Uranium has an atomic number of 92 which means there are 92 protons and 92 electrons in the atomic structure. U-238 has 146 neutrons in the nucleus, but the number of neutrons can vary from 141 to 146. Because uranium is radioactive, it is constantly emitting particles and changing into other elements. Uranium has a well-established radioactive decay series. U-238 and U-235 (which has 143 neutrons) are the most common isotopes of uranium. Uranium naturally contains all three isotopes (U-238, U-235 and U-234), and it rarely varies more than 0.01% from the average composition shown in the table below without our intervention.

Source: Wikimedia Commons

To produce fuel-grade uranium, the uranium has to be processed to produce uranium dioxide and to enrich or concentrate the U-235 in the fuel pellets. During this processing, depleted uranium (DU), enriched in U-238 and depleted in U-235, is produced. DU and enriched uranium have numerous civilian and military uses. Since U-235 is the most radioactive isotope of uranium, the removal of it to makes DU the least radioactive phase of uranium, but it still has heavy metal toxicity issues.

Despite any processing, enriched, depleted or natural uranium all behave the same chemically. Uranium ore oxidizes to uranium oxy-hydroxides when exposed to the air and/or water.

U-235, when bombarded by neutrons, fissions or splits into two smaller nuclei and releases energy and starts a nuclear chain reaction. Because of the energy released, U-235 is efficient for power generation and the only isotope of uranium that can sustain these reactions. For electrical power plants, this reaction is controlled so that it will not produce too many neutrons or heat and become explosive. Go to The Basics of Nuclear Reactions to find out more about nuclear reactions.

Because uranium decays by emitting alpha particles at a constant rate (half-life = time it takes for half of the material to decay to another substance), uranium minerals are very useful for dating rocks and determining the age of the earth.

Uses for Uranium

Besides nuclear power plant fuel, U has numerous other military and civilian uses. Uranium and other radionuclides are used by nuclear medicine (MRIs, X-rays, radioisotope injections, radiation treatment, etc.), scientific research (age-dating materials, compositional information, metabolic studies, etc.), agriculture (irradiating food and seeds), consumer products (smoke detectors, watches, irradiating bandages and other items to sterilize them, computer components, etc), materials testing for numerous industries (automotive, aircraft, construction, mining and oil) and space exploration (fuels). In addition, depleted uranium (DU) has been used for counterweights in sail boat's keel, ballast in boats, pigments and radiation shielding.

Depleted uranium is used by the military for its armour piercing capabilities as penetrators, and highly enriched (>90% U-235) Uranium is used by the military for weapons.

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