SUMMARY / RELATED TOPICS

Atomic number

The atomic number or proton number of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element, it is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is equal to the number of electrons; the sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom. Since protons and neutrons have the same mass and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units, is within 1% of the whole number A. Atoms with the same atomic number but different neutron numbers, hence different mass numbers, are known as isotopes. A little more than three-quarters of occurring elements exist as a mixture of isotopes, the average isotopic mass of an isotopic mixture for an element in a defined environment on Earth, determines the element's standard atomic weight.

It was these atomic weights of elements that were the quantities measurable by chemists in the 19th century. The conventional symbol Z comes from the German word Zahl meaning number, before the modern synthesis of ideas from chemistry and physics denoted an element's numerical place in the periodic table, whose order is but not consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was the nuclear charge and a physical characteristic of atoms, did the word Atomzahl come into common use in this context. Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, so they can be numbered in order. Dmitri Mendeleev claimed. However, in consideration of the elements' observed chemical properties, he changed the order and placed tellurium ahead of iodine; this placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time.

A simple numbering based on periodic table position was never satisfactory, however. Besides the case of iodine and tellurium several other pairs of elements were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties; however the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium onward. In 1911, Ernest Rutherford gave a model of the atom in which a central core held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms; this central charge would thus be half the atomic weight. In spite of Rutherford's estimation that gold had a central charge of about 100, a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was equal to its place in the periodic table.

This proved to be the case. The experimental position improved after research by Henry Moseley in 1913. Moseley, after discussions with Bohr, at the same lab, decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of Z. To do this, Moseley measured the wavelengths of the innermost photon transitions produced by the elements from aluminum to gold used as a series of movable anodic targets inside an x-ray tube; the square root of the frequency of these photons increased from one target to the next in an arithmetic progression. This led to the conclusion that the atomic number does correspond to the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.

After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen to uranium were examined by his method. There were seven elements which were not found and therefore identified as still undiscovered, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, all seven of these missing elements were discovered. By this time, the first four transuranium elements had been discovered, so that the periodic table was complete with no gaps as far as curium

12 cm Minenwerfer M 15

The 12 cm Minenwerfer M 15 was a medium mortar used by Austria-Hungary in World War I. It was designed by the Army's own Technisches und Administratives Militär-Komitee as an enlarged 9 cm Minenwerfer M 14 in 1915; the War Ministry decided to order 50 from the TMK, but the latter preferred only to produce 10 and switch the remaining 40 to the 14 cm Minenwerfer M 15, but no response was made by the Ministry. The TMZ placed an order for the 10 mortars from Teudloff & Dittrich in Vienna at the end of 1915. A follow on order for another hundred was canceled in February 1916. Ortner, M. Christian; the Austro-Hungarian Artillery From 1867 to 1918: Technology and Tactics. Vienna, Verlag Militaria, 2007 ISBN 978-3-902526-13-7

Lambda Pyxidis

Lambda Pyxidis is a yellow-hued star in the southern constellation of Pyxis. It is visible to the naked eye, having an apparent visual magnitude of 4.68. Based upon an annual parallax shift of 16.98 mas as seen from Earth, it is located around 192 light years from the Sun. Measurements of changes in the star's proper motion over time indicate this is an astrometric binary system; the visible component is an evolved G-type giant star with a stellar classification of G8.5 IIIb Fe−1 and a spectrum that displays an underabundance of iron with weak cyanogen lines. It is a red clump star, generating energy through the fusion of helium at its core. Lambda Pyxidis is an estimated 1.3 billion years old. It is radiating 49 times the Sun's luminosity from its photosphere at an effective temperature of 5,126 K