Remnants of hundreds of supernovae have been found. More recently, a second theory has proposed that uranium is created during the merger of two neutron stars. Neutron stars are very dense, with a teaspoon of neutron star material having a mass of the order of 5 billion tonnes.
When two such bodies come close together the intense gravitational forces cause them to violently merge, giving off gravitational waves and producing huge amounts of heavy elements, such as gold, platinum and uranium. So, we know that the Earth's uranium was produced through one or more of these processes, and that this material was inherited by the solar system of which the Earth is a part. We can calculate the abundances of U and U at the time the Earth was formed.
Knowing further that the production ratio of U to U in a supernova is about 1. This 'single stage' is, however, an oversimplification. In fact, multiple supernovae from over 6 billion to about million years ago were involved. Additionally, studies of the isotopic abundances of elements, such as silicon and carbon in meteorites, have shown that more than ten separate stellar sources were involved in the genesis of solar system material. Thus the relative abundance of U and U at the time of formation of the solar system:.
Many analyses have been made of the uranium in the rocks forming the continental and oceanic crusts, and in samples of the Earth's mantle exposed as uplifted slices in mountain belts or as 'xenoliths' in basalts and kimberlites hosts of diamonds.
We can have some confidence that these measurements are robust for the crust and upper mantle of the Earth, but less confidence that we know the abundance of uranium in the lower mantle and the outer and inner cores.
While on average the abundance of uranium in meteorites is about 0. Allowing for the extraction of a core-forming iron-nickel alloy with no uranium because of the characteristic of uranium which makes it combine more readily with minerals in crustal rocks rather than iron-rich ones , this still represents a roughly twofold enrichment in the materials forming the proto-Earth compared with average meteoritic materials.
The present-day abundance of uranium in the 'depleted' mantle exposed on the ocean floor is about 0. The continental crust, on the other hand, is relatively enriched in uranium at some 1. This represents a fold enrichment compared with the primitive mantle. In fact, the uranium lost from the 'depleted' oceanic mantle is mostly sequestered in the continental crust.
It is likely that the process or processes which transferred uranium from the mantle to the continental crust are complex and multi-step. However, for at least the past 2 billion years they have involved:.
All through this crust-forming cycle, the lithophile character of uranium is manifest in the constancy of the potassium to uranium ratio at about 10, in the rock range from peridotite to granite.
Because we would like to keep track of how uranium is distributed in the Earth, the abundance and isotopic characteristics of lead — the radiogenic daughter of U and U — are useful parameters. Table 1 below highlights the relatively low abundance of lead in the Earth's mantle and the consequent high uranium to lead ratio, compared with meteorites.
The difference in abundance can most likely be explained by lead's volatile nature and tendency to combine with iron, with lead being lost during terrestrial accretion and core separation. Pb is the final stable decay product of U, and Pb is that of U Pb is non-radiogenic. The figure given for the continental crust is an average of the entire crust. Of course, local concentration of uranium can far exceed these values, ranging up to 50 ppm disseminated in some granites, to much higher values in ore deposits.
In fact, in the geological past, local concentrations of uranium have occasionally achieved natural criticality, for example the Oklo reactors in Gabon see below. Convection in the outer core and the mantle, whereby heat is transferred by movement of heated matter, governs many of the Earth's endogenous processes.
The convection in the core may be driven by the heat released during progressive solidification of the core latent heat of crystallisation and leads to the self-sustaining terrestrial dynamo which is the source of the Earth's magnetic field. These plumes then ascend, essentially without gaining or losing heat, and undergo decompression melting close to the Earth's surface at 'hot spots' like Hawaii, Reunion and Samoa.
However, the primary source of energy driving the convection in the mantle is the radioactive decay of uranium, thorium and potassium. In the present Earth, most of the energy generated is from the decay of U ca. At the time of the Earth's formation, however, decay of both U and K would have been about equal in importance and both would have exceeded the heat production of U A simple way of viewing the process of plate tectonics — the formation and disposal of oceanic lithosphere — is that this is the mechanism by which the mantle sheds heat.
The Earth is well insulated thermally and the heat loss from the surface now can reflect heat generation a considerable time in the past. Measurements of heat have led to estimates that the Earth is generating between 30 and 44 terawatts of heat, much of it from radioactive decay. Natural abundance. Uranium occurs naturally in several minerals such as uranite pitchblende , brannerite and carnotite.
It is also found in phosphate rock and monazite sands. World production of uranium is about 41, tonnes per year. Extracted uranium is converted to the purified oxide, known as yellow-cake. Uranium metal can be prepared by reducing uranium halides with Group 1 or Group 2 metals, or by reducing uranium oxides with calcium or aluminium.
Help text not available for this section currently. Elements and Periodic Table History. In the Middle Ages, the mineral pitchblende uranium oxide, U 3 O 8 sometimes turned up in silver mines, and in Martin Heinrich Klaproth of Berlin investigated it. He dissolved it in nitric acid and precipitated a yellow compound when the solution was neutralised.
He realised it was the oxide of a new element and tried to produce the metal itself by heating the precipitate with charcoal, but failed.
The discovery that uranium was radioactive came only in when Henri Becquerel in Paris left a sample of uranium on top of an unexposed photographic plate. It caused this to become cloudy and he deduced that uranium was giving off invisible rays. Radioactivity had been discovered. Atomic data. Glossary Common oxidation states The oxidation state of an atom is a measure of the degree of oxidation of an atom. Oxidation states and isotopes. Glossary Data for this section been provided by the British Geological Survey.
Relative supply risk An integrated supply risk index from 1 very low risk to 10 very high risk. Recycling rate The percentage of a commodity which is recycled. Substitutability The availability of suitable substitutes for a given commodity.
Reserve distribution The percentage of the world reserves located in the country with the largest reserves. Political stability of top producer A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators. Political stability of top reserve holder A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.
Supply risk. Relative supply risk 5. Young's modulus A measure of the stiffness of a substance. Shear modulus A measure of how difficult it is to deform a material.
Bulk modulus A measure of how difficult it is to compress a substance. Vapour pressure A measure of the propensity of a substance to evaporate. Pressure and temperature data — advanced. Listen to Uranium Podcast Transcript :. You're listening to Chemistry in its element brought to you by Chemistry World , the magazine of the Royal Society of Chemistry.
For Chemistry in its element this week, can you guess what connects boat keels, armour piercing weaponry, beautiful coloured glass that you can track down with a geiger counter and more oxidation states than a chemist can shake a glass rod at.
If not, here's Polly Arnold with the answer. Uranium is certainly one of the most famous, or perhaps I should say infamous, elements. It is the heaviest naturally occurring element. It is actually more abundant in the earth's crust than silver. It is one of eight elements named in honour of celestial objects, but you might not think that uranium deserves to be named after the planet Uranus. The lustrous black powder that the chemist Klaproth isolated from the mineral pitchblende in - just eight years after Uranus was discovered - was in fact an oxide of uranium.
Samples of the metal tarnish rapidly in air, but if the metal is finely divided, it will burst into flames. Uranium sits amongst the actinides, the second shell of metals to fill their f-orbitals with valence electrons, making them large and weighty. Chemically, uranium is fascinating. Its nucleus is so full of protons and neutrons that it draws its core electron shells in close. This means relativistic effects come into play that affect the electron orbital energies.
The inner core s electrons move faster, and are drawn in to the heavy nucleus, shielding it better. So the outer valence orbitals are more shielded and expanded, and can form hybrid molecular orbitals that generated arguments over the precise ordering of bonding energies in the uranyl ion until as recently as this century. This means that a variety of orbitals can now be combined to make bonds, and from this, some very interesting compounds.
In the absence of air, uranium can display a wide range of oxidation states, unlike the lanthanides just above it, and it forms many deeply coloured complexes in its lower oxidation states. The uranium tetrachloride that Peligot reduced is a beautiful grass-green colour, while the triiodide is midnight-blue. Because of this, some regard it as a 'big transition metal'.
Most of these compounds are hard to make and characterise as they react so quickly with air and water, but there is still scope for big breakthroughs in this area of chemistry. The ramifications of relativistic effects on the energies of the bonding electrons has generated much excitement for us synthetic chemists, but unfortunately many headaches for experimental and computational chemists who are trying to understand how better to deal with our nuclear waste legacy.
In the environment, uranium invariably exists as a dioxide salt called the uranyl ion, in which it is tightly sandwiched between two oxygen atoms, in its highest oxidation state. Uranyl salts are notoriously unreactive at the oxygen atoms, and about half of all known uranium compounds contain this dioxo motif. One of the most interesting facets of this area of uranium chemistry has emerged in the last couple of years: A few research groups have found ways to stabilise the singly reduced uranyl ion, a fragment which was traditionally regarded as too unstable to isolate.
This ion is now beginning to show reactivity at its oxygen atoms, and may be able to teach us much about uranium's more radioactive and more reactive man-made sisters, neptunium and plutonium - these are also present in nuclear waste, but difficult to work with in greater than milligram quantities. Outside the chemistry lab, uranium is best known for its role as a nuclear fuel. It has been at the forefront of many chemists' consciousness over recent months due to the international debate on the role that nuclear power can play in a future as a low-carbon energy source, and whether our new generations of safer and efficient power stations are human-proof.
To make the fuel that is used to power reactors to generate electricity, naturally occurring uranium, which is almost all U, is enriched with the isotope U which is normally only present in about 0. The leftovers, called depleted uranium, or DU, have a much-reduced U content of only about 0.
Because it is so dense, DU is also used in shielding, in the keels of boats and more controversially, in the noses of armour-piercing weapons. The metal has the desirable ability to self-sharpen as it pierces a target, rather than mushrooming upon impact the way conventional tungsten carbide tipped weapons do. Critics of DU weaponry claim it can accumulate around battlefields. Because uranium is primarily an alpha-emitter, its radioactivity only really becomes a problem if it gets inside the body, where it can accumulate in the kidneys, causing damage.
However, uranium is also a heavy metal, and its chemical toxicity is of greater importance - it is approximately as toxic as lead or mercury. But uranium doesn't deserve it's image as one of the periodic table's nasties. Much of the internal heat of the earth is considered to be due to the decay of natural uranium and thorium deposits.
Perhaps those looking to improve the public image of nuclear power should demand the relabelling of geothermal ground-source heat pumps as nuclear? The reputation of this element would also be significantly better if only uranium glass was the element's most publicly known face.
In the same way that lead salts are added to glass to make sparkling crystal glassware, uranyl salts give a very beautiful and translucent yellow-green colour to glass, although glassmakers have experimented to produce a wide range of gem-like colours. An archaeological dig near Naples in unearthed a small green mosaic tile dated back to 79 AD, which was reported to contain uranium, but these claims have not been verified.
However in the early th and early 20 th century it was used widely in containers and wine-glasses. If you think that you own a piece, you can check with a Geiger counter, or by looking for the characteristic green fluorescence of the uranium when held under a UV-lamp. Pieces are generally regarded as safe to drink from, but you are advised not to drill holes in them, or wear them.
Fair enough. Or inadvertently eating it too, presumably. That was Edinburgh University chemist Polly Arnold explaining the softer side of the armour piercing element Uranium. Next week Andrea Sella will be introducing us to some crystals with intriguing properties.
You HAVE to see this. He beckoned me into a hallway. As the crystals caught the light from the new fluorescent lights hanging from the ceiling, the pink colour seemed to deepen and brighten up.
We moved the crystals back into the sunlight and the colour faded again, and moving the crystals back and forth they glowed and dimmed in magical fashion. But what did they contain? Well, the answer's Erbium and you can hear all about it in next week's Chemistry in its element. In a nuclear-fueled power plant, water is turned into steam, which drives turbine generators to produce electricity. The main difference between a nuclear power plant and a coal- or natural gas-fired power plant is the source of heat.
At a nuclear power plant, the heat to make the steam is created when uranium atoms split by a process called fission. There is no combustion in a nuclear reactor, as opposed to those powered by fossil fuels. Naturally occurring uranium is found as U Other isotopes of uranium are known but are very rare and usually short-lived.
Uranium decays slowly by emitting alpha particles. An alpha particle emitted from the uranium nucleus is positively charged and made up of two protons and two neutrons, which is physically and chemically identical to a helium nucleus. The U isotope is useful in dating the ages of some rocks and geologic events. When the nucleus of a U atom captures a moving neutron it splits in two fission reaction and releases energy in the form of heat and radiation, and two or three additional neutrons are expelled from the nucleus.
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