Naturally occurring element

Natural abundance. The natural abundances listed are on an atom percent basis for the stable nuclides present in naturally occurring elements in the earth s crust. 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

Apart from naturally occurring elements, there are now newly made elements beyond uranium. These constitute the transuranic series. All the elements in this series are radioactive. 

Few of the naturally occurring elements have significant amounts of radioactive isotopes, but there are many artificially produced radioactive species. Mass spectrometry can measure both radioactive and nonradioactive isotope ratios, but there are health and safety issues for the radioactive ones. However, modem isotope instmments are becoming so sensitive that only very small amounts of sample are needed. Where radioactive isotopes are a serious issue, the radioactive hazards can be minimized by using special inlet systems and ion pumps in place of rotary pumps for maintaining a vacuum. For example, mass spectrometry is now used in the analysis of Pu/ Pu ratios. 

Occurrence and Recovery. Rhenium is one of the least abundant of the naturally occurring elements. Various estimates of its abundance in Earth s cmst have been made. The most widely quoted figure is 0.027 atoms pet 10 atoms of silicon (0.05 ppm by wt) (3). However, this number, based on analyses for the most common rocks, ie, granites and basalts, has a high uncertainty. The abundance of rhenium in stony meteorites has been found to be approximately the same value. An average abundance in siderites is 0.5 ppm. In lunar materials, Re, when compared to Re, appears to be enriched by 1.4% to as much as 29%, relative to the terrestrial abundance. This may result from a nuclear reaction sequence beginning with neutron capture by tungsten-186, followed by p-decay of of a half-hfe of 24 h (4) (see Extraterrestrial materials). 

Plutonium (symbol Pu atomic number 93) is not a naturally occurring element. Plutonium is formed in a nuclear reaction from a fertile U-238 atom. Since U-238 is not fissile, it has a tendency to absorb a neutron in a reactor, rather than split apart into smaller fragments. By absorbing the extra neutron, U-238 becomes U-239. Uranium-239 is not very stable, and undergoes spontaneous radioactive decay to produce Pu-239. 

Ocean sea water is roughly equivalent in strength to a 3 j % w/v solution of sodium chloride, but it has a much more complex composition, embodying a number of major constituents, and traces at least of almost all naturally occurring elements. For convenience, however, the concentration of salts in any sample of sea water is expressed in terms of the chloride content, either as chlorinity or as salinity. Both these units are again subject to arbitrary definition and do not conform simply to the chemical composition. 

Nuclear activation analysis (NAA) is a method for qualitatively and quantitatively detg elemental compn by means of nuclear transmutations. The method involves the irradiation or bombardment of samples with nuclear particles or high-energy electromagnetic radiation for the specific purpose of creating radioactive isotopes from the stable or naturally-occurring elements present. From the numbers, types and quantities of radioactive elements or radionuclides, it is possible to deduce information about the elemental compn of the original sample... 

The last forty years have seen an extensive, world-wide investigation of the chemical properties of the synthetic element, plutonium. As a result, as much is known about the chemical properties of this element as is known about the chemical properties of most of the naturally occurring elements. The papers in this volume, presented at the Symposium on the Chemistry of Plutonium held during the Kansas City meeting of the American Chemical Society, in September, 1982, represent an up-dating of this large amount of information. 

The periodic table lists all the known elements in numerical order, starting with the lightest (hydrogen) and proceeding to the heaviest (uranium, among naturally occurring elements). The list is broken into seven rows. 

The molar mass of any naturally occurring element is the sum of the contributions from its isotopes. 

Chemicals are classed as either elements or compounds. The former are substances which caimotbe split into simpler chemicals, e.g. copper. There are 90 naturally-occurring elements and 17 artificially produced. In nature the atoms of some elements can exist on their own, e.g. gold, whilst in others they fink with other atoms of the same element to form molecules, e.g. two hydrogen atoms combine to form a molecule of hydrogen. Atoms of different elements can combine in simple numerical proportions 1 1, 1 2, 1 3, etc. to produce compounds, e.g. copper and oxygen combine to produce copper oxide hydrogen and oxygen combine to produce water. Compounds are therefore chemical substances which may be broken down to produce more than one element. Molecules are the smallest unit of a compound. 

Polonium, completing the elements of Group 16, is radioactive and one of the rarest naturally occurring elements (about 3 x 10 " % of the Earth s crust). The main natural source of polonium is uranium ores, which contain about lO g of Po per ton. The isotope 210-Po, occurring in uranium (and also thorium) minerals as an intermediate in the radioactive decay series, was discovered by M. S. Curie in 1898. 

Two centuries later, chemists had identified 63 of the 92 naturally occurring elements. But they had no useful way of organizing them, no system that would allow them to understand the elements relationship to one other. Did the elements have any order The question stumped the world s best chemists until the Russian scientist Dmitri Mendeleyev solved the problem in 1869. His eureka moment did not come in his lab but in his bed. I saw in a dream, he wrote, a table where all the elements fell into place... 

Rhenium (75) was discovered in 1925 by Ida Tacke and Walter Noddack as the last naturally occurring element. The first artificially produced element was identified by Emilio G. Segre in 1937. Ernest Lawrence detected technetium in a molybdenum sample, which he had bombarded in his cyclotron. All elements discovered since then have been generated artificially. 

Scientific chemistry has its roots in the European Enlightenment. All 92 naturally occurring elements were discovered and identified here. The map shows that England, France, and Sweden played central roles, whereas in Germany research was carried out in the various regional centers. With the advent of atomic research, the emphasis on the discovery of the artificial elements shifted to the USA. They were later joined by Russia (Dubna) and Germany (Institute for Heavy-Ion Research). 

Name from the continent Europe, where all naturally occurring elements were discovered... 

M Astatine is isolated in tiny amounts from reactor materials. The Bohr atomic model shows the tightly packed electron shell. One can formally see" the instability. It was the last of the 92 naturally occurring elements to be found. 

Most naturally occurring elements are mixtures of two or more isotopes in which just one predominates. Isotopes can also be prepared... 

Radiological Background. The surface of the earth contains a very heterogeneous distribution of the naturally occurring elements. 

Mercury is a naturally occurring element. Natural emissions of mercury, e.g. from ore deposits and from volcanic activity, are variously estimated at amounts between 2500 and 5500 tonnes/year and are thus similar in magnitude to anthropogenic emissions, which are currently estimated at some 3600-4100 tonnes/year world-wide. Some 30000 tonnes of mercury are readily available in the environment, i.e. in the atmosphere or in the mixing zone of the oceans, with tens of millions of tonnes in the upper layers of the continental masses and still more in the deep oceans (see Table 2.1). 

The number of stable isotopes for the naturally occurring elements tends to increase with increasing atomic number, to a maximum of 10 for Sn (Fig. 1). Elements with low atomic numbers tend to have the lowest number of stable isotopes, limiting the possible ways in which isotopic compositions can be reported. Both H and C have only two stable isotopes (Fig. 1), and therefore isotopic compositions are reported using one ratio, D/H and C/ C, respectively. Single ratios can only be used for B and N, and data are reported as "B/ B and W N, respectively. Of the non-traditional stable isotope systems discussed in this volume, only three have just two stable isotopes (Li, Cl, and Cu Fig. 1). 

About two-thirds of all the naturally occurring elements are metals. 

Virtually every naturally occurring element has been detected in seawater. Those that are present at concentrations less than 100 p,mol/kg are termed trace elements. As shown in Figure 11.1, the trace elements are mostly metals and metalloids. (Oceanographers tend to use the terms trace metals and trace elements interchangeably.)... 

The superscript nat indicates a material with a ratio of isotopes identical to the isotopic ratio for the naturally occurring element. 

Mineral A naturally occurring element or compound defined by its chemical composition and crystal structure (see chapter 2). By custom in recent times, mineral species have been given names ending in ite. Each year, about 100 new species are proposed, and usually about half are accepted by the International Mineralogical Association Commission on New Names as bona fide new species. The Glossary of Mineral Species (Fleischer, 1987) is a current source of information. Updates are published annually in the Mineralogical Record. 

Table Group VIIB (the halogens), is the earth s rarest naturally occurring element. All its isotopes are radioactive (Table I), hence the Greek name aoTaTCoC, meaning unstable 44, 45). The possibility of their existence was predicted from the -decay of polonium (55). Its three naturally occurring isotopes, At, At, and At, are the extremely short-lived natural daughters of AcA (77), RaA (76,173), and AcK (72), respectively.

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