Early Atomic Theory and Structure

Lightning occurs when electrons move to neutralize a charge difference between the clouds and the Earth. 

Hie Nuclear Atom Isotopes of the Elements Atomic Mass 

Pure substances are classified as elements or compounds, but just what makes a substance possess its unique properties How small a piece of salt will still taste salty Carbon dioxide puts out fires, is used by plants to produce oxygen, and forms dry ice when solidified. But how small a mass of this material still behaves like carbon dioxide Substances are in their simplest identifiable form at the atomic, ionic, or molecular level. Further division produces a loss of characteristic properties. 

What particles lie within an atom or ion How are these tiny particles alike How do they differ How far can we continue to divide them Alchemists began the quest, early chemists laid the foundation, and modem chemists continue to build and expand on models of the atom. 

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Early Atomic Theory and Structure 81 17 Oxidation-Reduction 411... 

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... 

The observation of atomic spectra stimulated physicists in the early 19th century to develop the theory of quantum mechanics. This theory sets out to explain all physical phenomena at an atomic scale and atomic spectroscopy is an important validation. Quantum mechanics is flawed, however, notably in the description of gravity, but it is the best theory at present (although super string theory promises well) for the description of the structure of nuclei, atoms and molecules. 

A SHORT HISTORY OF CHEMISTRY (3rd edition), J.R. Partington. Classic exposition explores origins of chemistry, alchemy, early medical chemistry, nature of atmosphere, theory of valency, laws and structure of atomic theory, much more. 428pp. 5X x 8X. (Available in U.S. only) 65977-1 Pa. 10.95... 

The solution of a protein crystal structure can still be a lengthy process, even when crystals are available, because of the phase problem. In contrast, small molecule (< 100 atoms) structures can be solved routinely by direct methods. In the early fifties it was shown that certain mathematical relationships exist between the phases and the amplitudes of the structure factors if it is assumed that the electron density is positive and atoms are resolved [255]. These mathematical methods have been developed [256,257] so that it is possible to solve a small molecule structure directly from the intensity data [258]. For example, the crystal structure of gramicidin S [259] (a cyclic polypeptide of 10 amino acids, 92 atoms) has been solved using the computer programme MULTAN. Traditional direct methods are not applicable to protein structures, partly because the diffraction data seldom extend to atomic resolution. Recently, a new method derived from information theory and based on the maximum entropy (minimum information) principle has been developed. In the immediate future the application will require an approximate starting phase set. However, the method has the potential for an ab initio structure determination from the measured intensities and a very small sub-set of starting phases, once the formidable problems in providing numerical methods for the solution of the fundamental equations have been solved. 

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