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Isobel A

on 22 October 2013

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The Early Pioneers
1817- Johan Doberiner-The Law of Triads
1862-Alexander de Chancourtios-Telleric Helix
1863- John Newlands-Law of Octaves
1864- Julius Meyer-Horizontal Periodic Table
1869- The First Periodic Table
1911- Rutherford Model of the Atom
The first periodic table was designed by Dimitri Mendeleev, a Russian chemist. He arranged groups of elements with similar chemical properties so that they fell into vertical columns in a table. However not all elements fitted this pattern neatly. Mendeleev's solution was to move certain elements to different positions, despite their accepted atomic weight, in order to group them with elements with similar properties.
Mendeleev is often considered to be the
"father of the periodic table"
1890-1910 : Discovery of The Noble Gases
William Ramsay investigated that nitrogen made by removal of other known components from the air had a slightly different density to nitrogen made by chemical decomposition. First he discovered Argon and then he predicted a complete family of elements between group 7 and 1 of the periodic table, which is now called the Noble Gases.With the fractional distillation of liquefied air, he and Morris Travers discovered Neon, Krypton and Xenon.He also isolated Helium, which had previously only been observed in the spectrum of the sun, and in 1910 made and characterised Radon. Ramsay received a Nobel Prize in Chemistry in 1910.
1896 - Discovery of Radioactivity
Radioactivity was first discovered by the French scientist Henri Bequerel, while working on phospherent materials. He tried wrapping a photographic glass plate in black paper and placing various phospherent materials on them. All results were negative until he tried using uranium salts. The result with these compounds was a deep blackening of the plate. However, it soon became clear that the blackening of the plate had nothing to do with phospherence, because the plate blackened when the uramium was kept in the dark, and also when non-phospherent uranium and even metallic uranium blackened the plate. Evidently, there was a new form of radiation that could pass through paper that was causing the plate to blacken.
1898 - Marie and Pierre Curie
Marie Curie was born in 1867,in Poland. She earned her degree for physics in France and was fascinated by the rays that were recently discovered by the scientists Henri Becqerul and Wilhelm Roentgen. Marie and her husband spent many hours in the laboratory investigating pitchblende, which was a material Marie suspected contained a new, undiscovered element. They eventually discovered that there were not one, but two elements in the pitchblende! Marie named one of the elements Polonium, after her homeland. She named the other Radium, because it emitted strong rays.
In 1911,Ernest Rutherford gave a model of an atom in which a central core held the majority of the atom's mass(which is the mass of an atom at rest.It describes a single isotope.) and a positive charge,which was to be approximately equal to half of the elements atomic weight,(an average mass of all the element's isotopes) expressed in numbers of hydrogen atoms(although his estamation of gold's central charge was almost 25% different from the atomic number of gold, the single element Rutherford made his guess from).Rutherford estimated that the central core of an atom of gold had a mass of 100 and the orbiting electrons made up the rest of the weight. The atomic weight of gold was 197, so he made a very close guess!
1911 - Antonius van Den Broek
Antonius van den Broek first suggested that the central charge(the protons in the core) and the number of electrons in an atom was exactly equal to the atomic number 'Z'. This eventually proved to be correct.
1913 - Bohr Model of the Atom
The Bohr model was introduced by Neils Bohr in 1913, and depicts the atom as small positively charged nucleus surrounded by electrons that travel in orbits-like the Solar System in structure.This model proves that the chemical properties of each element depends on the number of electrons in the outer orbit of the atom.The model's key success was found in explaining the Rydeberg formula for predicting the spectral emission lines for atomic electrons moving energy levels.

"Everything we call real is made of things that cannot be regarded a real"
Neil Bohr
1913 - Moseley's Law
Henry Moseley devised an experiment to test Bohr's theory that the frequency of spectral lines is proportional to the square of the atomic number "Z". Moseley's experiment put elements from aluminium(Z=13) to Gold (Z=79) inside an x-ray tube. The square root of the frequency of these x-rays increased from one target to the next in a linear fashion,corresponding to the atomic number. This led to the conclusion that the atomic number closely matches the calculated electric charge of the nucelus.
1932 - The Discovery of the Neutron
Rutherford continued his experiments from 1915 to 1917. He was trying to understand why the nuclei of heavy atoms had more than twice as much mass as you would expect if they were made of multiples of hydrogen nuclei. Rutherford developed his theory of nuclear electrons to explain the extra mass - he thought there were extra protons in the nucleus, which were partially balanced by 'nuclear electrons'.
This thinking changed when Chadwick discovered a third type of atomic particle called the
. Neutrons had mass but no electrical charge. It was the neutrons that were providing the extra atomic mass.
1945-Today : The Discovery of the Transuranium Elements
The Transuranium elements are the chemical elements with atomic numbers greater than 92 (the atomic number of uranium). All of these elements are unstable, and decay radioactively into other elements. All of the transuranium elements have been discovered scince 1945, at just 3 laboratories around the world :
1) Lawrence Berkeley National Laboratory, California
2) Society For Heavy Ion Research, Darmstadt, Germany
3) The Joint Institute for Nuclear Research, Dubna, Russia

There was, I think, a feeling that the best science was that done in the simplest way. In experimental work, as in mathematics, there was 'style' and a result obtained with simple equipment was more elegant than one obtained with complicated apparatus, just as a mathematical proof derived neatly was better than one involving laborious calculations. Rutherford's first disintegration experiment, and Chadwick's discovery of the neutron had a 'style' that is different from that of experiments made with giant accelerators.
— John Ashworth Ratcliffe
All scientists build on the work of previous scientists. Good scientists discover new information and make sense of it, linking it to other ideas.
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