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 The Atomic Theory

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مُساهمةموضوع: The Atomic Theory   الثلاثاء سبتمبر 21, 2010 12:01 pm

Rutherford's gold foil experiment, performed in conjunction with Geiger andMarsden, provided evidence for the nucleus due to the scattering of alphaparticles. The repulsion of some alpha particles suggested that the nucleusis positively charged, containing protons. Further work by Chadwick revealedthe existence of neutrons within the nucleus of the atom. The atomic numberdescribes the number of protons in the nucleus. For a neutral atom this isalso the number of electrons outside the nucleus. Subtracting the atomicnumber from the atomic mass number gives the number of neutrons in the nucleus.Isotopes are atoms of the same element (i.e., they have the same number ofprotons, or the same atomic number) which have a different number of neutronsin the nucleus. Isotopes of an element have similar chemical properties.Radioactive isotopes are called radioisotopes. Most of the elements in theperiodic table have several isotopes, found in varying proportions for anygiven element. The average atomic mass of an element takes into account therelative proportions of its isotopes found in nature. A nuclear binding forceholds the nucleus of the atom together. The nuclear mass defect, a slightlylower mass of the nucleus compared to the sum of the masses of its constituentmatter, is due to the nuclear binding energy holding the nucleus together.The mass defect can be used to calculate the nuclear binding energy, withE = mc2. The average binding energyper nucleon is a measure of nuclear stability. The higher the average bindingenergy, the more stable the nucleus.
The Bohr's model of the atom described the electrons as orbiting in discrete,precisely defined circular orbits. Electrons can only occupy certain allowedorbitals. For an electron to occupy an allowed orbit, a certain amount ofenergy must be available. Each orbit is assigned a quantum number, with thelowest quantum numbers being assigned to those orbitals closest to the nucleus.Only a specified maximum number of electrons can occupy an orbital. Undernormal circumstances, electrons occupy the lowest energy level orbitals closestto the nucleus. By absorbing additional energy, electrons can be promotedto higher orbitals, and release that energy when they return back to lowerenergy levels. The Bohr's model of the atom helped to offer one possibleexplanation for the emission spectrum formed by hydrogen and other gases.Photons are used to describe the wave-particle duality of light. The energyof a photon depends upon its frequency. This helps to explain the photoelectriceffect; only photons having a sufficiently high energy are capable of dislodgingan electron from the illuminated surface.E = hv where E is the photonenergy in J, v is the photon frequencyin Hz, and h is Planck's constant,6.626 × 10-34 J / Hz.Quantum theory offers a mathematical model to help explain the nature of the atom.Quantum theory describes a region surrounding the nucleus which has the highestprobability of locating an electron. These orbital "clouds" have some unusual andinteresting shapes.
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The Atomic Theory
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