Artificial elements, mass numbers

I Atomic mass and mass number are not the same. Atomic mass refers to the naturally occurring mixture of isotopes mass number refers to an individual isotope. Atomic mass is an average and is never an exact integer mass number is a sum (of the number of protons plus the number of neutrons) and is always an integer. Except for the artificial elements, mass numbers are not given in the periodic table. 

Symbol Cs atomic number 55 atomic weight 132.905 a Group lA (Group 1) alkali metal element electron configuration [Xe]6si atomic radius 2.65 A ionic radius (Cs ) 1.84 A ionization potential 3.89 eV valence +1 natural isotope Cs-133 37 artificial isotopes ranging in mass numbers from 112 to 148 and half-lives 17 microseconds (Cs-113) to 2.3x10 years (Cs-135). 

Symbol Lu atomic number 71 atomic weight 174.97 a lanthanide series element an /-block inner-transition metal electron configuration [Xe]4/i45di6s2 valence -1-3 atomic radius (coordination number 12) 1.7349A ionic radius (Lu3+) 0.85A two naturally-occurring isotopes Lu-176 (97.1%) and Lu-175(2.59%) Lu-172 is radioactive with a half-life of 4xl0i° years (beta-emission) several artificial isotopes known, that have mass numbers 155, 156, 167—174, 177—180. 

Isotopes are atoms of the same elements having different mass numbers. Radioisotopes are the isotopes of an element that are radioactive or emit ionizing radiation. All elements are known to form artificial radioactive isotopes by nuclear bombardment. 

Atomic masses are based on the relative atomic mass of 12C=12. These values apply to the elements as they exist in materials of terrestrial origin and to certain artificial elements. Values in parenthesis are the mass number of the isotope of the longest half-life. 

Technetium was the first of the artificially produced elements (1937) when it was obtained, as the isotopes 95Tc and Tc, by bombarding Mo with deuterons. Today twenty-one isotopes, all radioactive, are known with mass numbers 90-111. The longest lived isotope is Tc (ha 4 x 106 y), but the commonest is "Tc (tm — 2 x 105 y). It is isolated in fairly large quantities from spent nuclear fuel, where it constitutes ca. 6% of the fission products. The total amount of "Tc is about 78 tons, which exceeds the known amount of rhenium in the earth s crust. "Tc emits a soft (293.6 Kev) /3 particle and can be handled with only very modest precautions. 

Unfortunately, the produced amoimt of element 61 was too small to study its properties. Pool and Quill were nevertheless convinced that they had s)mthesized an isotope of element 61 with mass number 144 and half-life of 12.5 h. More isotopes of element 61, with mass number 144,147, and 149, were produced two years later in collaboration with Kurbatov, Law and MacDonald (Kurbatov et al., 1942 Law et al., 1941). Pool and his team decided to name the element cyclonium (symbol Cy) in honor of the cyclotron in which all artificial elements had been formed. Most chemists however questioned the validity of their assertions, and doubted that the neodymium targets had been entirely pure. Every presence of impurities... 

Forty-one isotopes of francium have been produced artificially. The most stable is francium-223. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element s name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. 

CAS 7723-14-0. P. Nonmetallic element of atomic number 15, group VA of periodic table, aw 30.97376, valences of 1,3,4,5 allotropes white (or yellow), red, and black phosphorus. No stable isotopes, several artificial radioactive isotopes with mass numbers 29-34. 

Elements also are transmuted into other elements by nuclear fission and fusion. Fission is the breakup of very large nuclei (at least as heavy as uranium) into smaller nuclei, as in the fission of U-236 in the following reaction 22f U IE Kr + 12 Ba + 3n, where n is the symbol for a neutron (charge = 0, mass number = +1). In fusion, nuclei combine to form larger nuclei, as in the fusion of hydrogen isotopes to make helium. Energy may also be released during both fission and fusion. These events may occur naturally—fusion is the process that powers the Sun and all other stars—or they may be made to occur artificially. 

Nuclides of the same element possess the same number of protons and electrons but may have different mass numbers the number of protons and electrons defines the element but the number of neutrons may vary. Nuchdes of a particular element that differ in the number of neutrons and, therefore, their mass number, are called isotopes (see Appendix 5). Isotopes of some elements occur naturally while others may be produced artificially. 

All atomic mass numbers from 1 to 238 are found naturally on earth except for masses 5 and 8. About 285 relatively stable and 67 naturally radioactive isotopes occur on earth totaling 352. In addition, the neutron, technetium, promethium, and the transuranic elements (lying beyond uranium) have now been produced artificially. In June 1999, scientists at the Lawrence Berkeley National Laboratory reported that they had found evidence of an isotope of Element 118 and its immediate decay... 

Isotopes of many lighter elements with lower mass numbers are radioactive, too. At least one radioisotope is known for every element. Most of these do not occur in nature but can be generated in reactors by nuclear reactions. Tritium (jH) is a naturally occurring radioisotope of hydrogen. Artificial radioactive isotopes are known for a number of elements. 

All atomic mass numbers from 1 to 238 are found naturally on earth except for masses 5 and 8. About 285 relatively stable and 67 naturally radioactive isotopes occur on earth totahng 352. In addition, the neutron, technetium, promethium, and the transuranic elements Oying beyond uranium) have now been produced artificially. In lune 1999, scientists at the Lawrence Berkeley National Laboratory reported that they had found evidence of an isotope of Element 118 and its immediate decay products of Elements 116,114, and 112. This sequence of events tended to reinforce the theory that was predicted since the 1970s that an island of stability existed for nuclei with approximately 114 protons and 184 neutrons. This island refers to nuclei in which the decay lasts for a period of time instead of a decay that occurs instantaneously. However, on luly 27,2001, researchers at LBNL reported that their laboratory and the facilities at the GSI Laboratory in Germany and at J anese laboratories failed to confirm the results of their earlier experiments where the fusion of a krypton atom with a lead target resulted in Element 118, with chains of decay leading to Elements 116, 114, and 112, and on down to Element 106. Therefore, the discovery was reported to be spurious. However, with the announcement it was said that different... 

Materials which will undergo fission and can be used to sustain a chain reaction are, in the nomenclature of the NRC, special nuclear materials. These include the isotopes of uranium with mass numbers 233 and 235, materials emiched in these isotopes, or the artificially made element, plutonium. Materials which have uranium or thorium, which also has a fissionable isotope, in them to the extent of 0.05% are called source materials. 

As if trying to restore the honour of American science after its setback in 1926 two physicists from the University of Ohio conducted the first experiment on artificial synthesis of element 61 in 1938. They bombarded a neodymium target with fast deutrons (the nuclei of heavy hydrogen). They believed that the resulting nuclear reaction Nd d- -- 61 -1- ra gave rise to an isotope of element 61. Their results were inconclusive but nevertheless they thought that they obtained an isotope of the new element with the mass number of 144 and the half-life of 12.5 hours. 

The development of the cyclotron and, later, the fission reactor gave the means for a variety of artificial transmutations, but it often was difficult to identify the element and mass number of a radioactive product. In many cases, individual radionuclides could be characterized only by simple features, such as half-life or attenuation of radiations in absorbers, which did not allow discriminating the components of a complex mixture. Chemical evidence was required in order to make definite identification with a particular element. 

Isotope Atoms of the same element (all chemically identical) having the same atomic number but containing different numbers of neutrons, giving different mass number. Some elements occur naturally as a mixture of different isotopes. All elements can produce radio isotopes artificially. 

Radioactive nuclei emit a particles, 13 particles, positrons, or y rays. The equation for a nuclear reaction includes the particles emitted, and both the mass numbers and the atomic numbers must balance. Uranium-238 is the parent of a natural radioactive decay series. A number of radioactive isotopes, such as and C, can be used to date objects. Artificially radioactive elements are created by the bombardment of other elements by accelerated neutrons, protons, or a particles. Nuclear fission is the splitting of a large nucleus into smaller nuclei plus neutrons. When these neutrons are captured efficiently by other nuclei, an uncontrollable chain reaction can occur. Nuclear reactors use the heat... 

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