Atomic number, discovery

Atomic number Element CAS Registry Number Symbol Atomic weight Discoverers and date of discovery... 

The elements beyond the actinides in the Periodic Table can be termed the transactinides. These begin with the element having atomic number 104 and extend, in principle, indefinitely. Although only six such elements, numbers 104—109, were definitely known in 1991, there are good prospects for the discovery of a number of additional elements just beyond number 109 or in the region of larger atomic numbers. They are synthesized by the bombardment of heavy nucHdes with heavy ions. 

Gallium [7440-55-3] atomic number 31, was discovered through a study of its spectral properties in 1875 by P. E. Lecoq de Boisbaudran and named from Gallia in honor of its discoverer s homeland. The first element to be discovered after the pubHcation of Mendeleev s Periodic Table, its discovery constituted a confirmation of the Table which was reinforced shordy after by the discoveries of scandium and germanium. 

By this time, the Periodic Table of elements was well developed, although it was considered a function of the atomic mass rather than atomic number. Before the discovery of radioactivity, it had been estabUshed that each natural element had a unique mass thus it was assumed that each element was made up of only one type of atom. Some of the radioactivities found in both the uranium and thorium decays had similar chemical properties, but because these had different half-Hves it was assumed that there were different elements. It became clear, however, that if all the different radioactivities from uranium and thorium were separate elements, there would be too many to fit into the Periodic Table. 

The stmcture of the particles inside the nucleus was the next question to be addressed. One step in this direction was the discovery of the neutron in 1932 by Chadwick, and the deterrnination that the nucleus was made up of positively charged protons and uncharged neutrons. The number of protons in the nucleus is known as the atomic number, Z. The number of neutrons is denoted by A/, and the atomic mass is thus A = Z - - N. Another step toward describing the particles inside the nucleus was the introduction of two forces, namely the strong force that holds the protons and neutrons together in spite of the repulsion between the positive charges of the protons, and the weak force that produces the transmutation by P decay. 

The discovery of hafnium was one of chemistry s more controversial episodes. In 1911 G. Urbain, the French chemist and authority on rare earths , claimed to have isolated the element of atomic number 72 from a sample of rare-earth residues, and named it celtium. With hindsight, and more especially with an understanding of the consequences of H. G. J. Moseley s and N. Bohr s work on atomic structure, it now seems very unlikely that element 72 could have been found in the necessary concentrations along with rare earths. But this knowledge was lacking in the early part of the century and, indeed, in 1922 Urbain and A. Dauvillier claimed to have X-ray evidence to support the discovery. However, by that time Niels Bohr had developed his atomic theory and so was confident that element 72 would be a... 

I became increasingly interested in triads of elements, partly because of their historical importance. As I claimed in my book, the discovery of atomic weight triads represents the first major hint that there exists some regularity that underlies the elements.21 Triads represent the first hint of a systematic and quantitative foundation between the numerical properties of the elements. Now since atomic weight was replaced by atomic number, it is... 

For example, whichever form of the table is used, an interesting feature emerges The sequence 2,10,18, 36,54, 86 of atomic numbers, in which each period is dosed in the sense of reaching a noble-gas structure, does not appear to have a strictly quantum-mechanical explanation. Although Pauli s brilliant discovery... 

As suggested in the title of the present article, we believe that the periodic table, which initially arose from the discovery of atomic weight triads, can now be further enhanced by recognizing the fundamental importance of atomic number triads. In addition one should recognize the more fundamental nature of the elements as basic substances rather than as simple substances, and that the periodic system is primarily a classification of the former. Whereas we previously suggested that these aims were best served by the left-step table we now favor the revised left-step table shown in Figure 3. 

As happens so often in science, a new and more precise technique of measurement led to a major discovery. When scientists first used mass spectrometers they found—much to their surprise—that not all the atoms of a single element have the same mass. In a sample of perfectly pure neon, for example, most of the atoms have mass 3.32 X 10-26 kg, which is about 20 times as great as the mass of a hydrogen atom. Some neon atoms, however, are found to be about 22 times as heavy as hydrogen. Others are about 21 times as heavy (Fig. B.6). All three types of atoms have the same atomic number so they are definitely atoms of neon. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

After Chadwick s discovery, scientists knew the three components of an atom protons and neutrons in the nucleus with electrons hovering outside. The masses and charges of these constituents are shown in Table 3.1. Chemists have developed a system to describe the elements based on their atomic makeup. The atomic number of an atom is the number of protons in the nucleus. This number is usually represented by the letter Z. Thus, for hydrogen Z = 1, for helium Z = 2, and so on. 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Astatine - the atomic number is 85 and the chemical symbol is At. The name derives from the Greek astatos for unstable since it is an unstable element. It was first thought to have been discovered in nature in 1931 and was named alabamine. When it was determined that there are no stable nuclides of this element in nature, that claim was discarded. It was later shown that astatine had been synthesized by the physicists Dale R. Corson, K. R. Mackenzie and Emilio Segre at the University of California lab in Berkeley, California in 1940 who bombarded bismuth with alpha particles, in the reaction Bi ( He, 2n ) "At. Independently, a claim about finding some x-ray lines of astatine was the basis for claiming discovery of an element helvetium, which was made in Bern, Switzerland. However, the very short half-life precluded any chemical separation and identification. The longest half-life associated with this unstable element is 8.1 hour °At. 

Bromine - the atomic number is 35 and the chemical symbol is Br. The name derives from the Greek bromos for stench or bad odor . It was first prepared by the German chemist Carl Ldwig in 1825 but it was first publically announced in 1826 by Balard and so the discovery is therefore credited to the French chemist and pharmacist Antoine-Jerome Balard. 

Dysprosium - the atomic number is 66 and the chemical symbol is Dy. The name derives from the Greek dysprositos for hard to get at , due to the difficulty in separating this rare earth element from a holmium mineral in which it was found. Discovery was first claimed by the Swiss chemist Marc Delafontaine in the mineral samarskite in 1878 and he called it philippia. Philippia was subsequently found to be a mixture of terbium and erbium. Dysprosium was later discovered in a holmium sample by the French chemist Paul-Emile Lecoq de Boisbaudron in 1886, who was then credited with the discovery. It was first isolated by the French chemist George Urbain in 1906. 

Francium - the atomic number is 87 and the chemical symbol is Fr. The name derives from the country France , where the French physicist Marguerite Percy from the Curie Institute in Paris, France discovered it in 1939 in the alpha particle decay of actinium, Ac => He => Fr, which was known as actinium-K and has a half-life of 22 minutes. An earlier claim of discovery in 1930 with the element name Virginium was determined to be incorrect. A similar claim for discovery of the element with atomic number 87 and named moldavium was also determined to be incorrect. The longest half-life associated with this unstable element is 22 minute Fr. 

Iridium - the atomic number is 77 and the chemical symbol is Ir. The name derives from the Latin Iris, the greek goddess of rainbows because of the variety of colors in the element s salt solutions . Iridium and osmium were both discovered in a crude platinum ore in 1803 by the English chemist Smithson Tennant. Iridium was discovered independently by the French chemist H. V. Collet-Descotils also in 1803. Descotils actually published one month before Tennant but Tennent is given credit for the discovery, perhaps because he alone also found osmium in the ore. 

Ruthenium - the atomic number is 44 and the chemcial symbol is Ru. The name derives from the Latin ruthenia for the old name of Russia . It was discovered in a crude platinum ore by the Russian chemist Gottfried Wilhelm Osann in 1828. Osann thought that he had found three new metals in the sample, pluranium, ruthenium and polinium.He later withdrew his claim of discovery. In 1844 the Russian chemist Karl Karlovich Klaus was able to show that Osann s mistake was due to the impurity of the sample but Klaus was able to isolate the ruthenium metal and he retained Osann s original name of ruthenium. 

Uranium - the atomic number is 92 and the chemical symbol is U. The name derives from the planet Uranus, which in Roman mythology was Father Heaven . The German chemist Martin-Heinrich Klaproth discovered the element in 1789, following the German/English astronomer William Hershel s discovery of the planet in 1781. The metal was first isolated by the French chemist Eugene-Melchior Peligot in 1841. 

Germany, Russia, and the United States were all involved in the synthetic production of element 110 and those elements of higher atomic numbers. This search has been an ongoing international effort by Peter Armbruster s team that in 1994 claimed to have discovered element 110 in their laboratory. (See Properties and Gharacteristics section for more on this discovery.)... 

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