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الموضوع: Nuclear Chemistry

  1. #1
    عضو ذهبي الصورة الرمزية زهراء غانم
    تاريخ التسجيل
    Jul 2011
    الدولة
    الاردن - ماركا
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    افتراضي Nuclear Chemistry



    Discovery of , � & Particles
    The discovery of x-rays by William Conrad Roentgen in November of 1895 excited the imagination of a generation of scientists who rushed to study this phenomenon. Within a few months, Henri Becquerel found that both uranium metal and salts of this element gave off a different form of radiation, which could also pass through solids. By 1898, Marie Curie found that compounds of thorium were also "radioactive." After pain-staking effort she eventually isolated two more radioactive elementspolonium and radiumfrom ores that contained uranium.
    In 1899 Ernest Rutherford found that there were at least two different forms of radioactivity when he studied the absorption of radioactivity by thin sheets of metal foil. One, which he called alpha () particles, were absorbed by metal foil that was a few hundredths of a centimeter thick. The other, beta (� particles, could pass through 100 times as much metal foil before they became absorbed. Shortly thereafter, a third form of radiation, gamma () rays, was discovered that could penetrate as much as several centimeters of lead.
    The results of early experiments on these three forms of radiation are shown in the figure below. The direction in which -particles were deflected by an electric field suggested that they were positively charged. The magnitude of this deflection suggested that they had the same charge-to-mass ratio as an He2+ ion. To test the equivalence between -particles and He2+ ions, Rutherford built an apparatus that allowed -particles to pass through a very thin glass wall into an evacuated flask that contained a pair of metal electrodes. After a few days, he connected these electrodes to a battery and noted that the gas in the flask did indeed give off the characteristic emission spectrum of helium.
    The effect of an electric field on -, �, and -radiation.



    Experiments with electric and magnetic fields demonstrated that �/i>-particles were negatively charged. Furthermore, they had the same charge-to-mass ratio as an electron. To date, no detectable difference has been found between �/i>-particles and electrons. The only reason to retain the name "�/i>-particle"is to emphasize the fact that these particles are ejected from the nucleus of an atom when it undergoes radioactive decay.
    The fact that -rays are not deflected by either electric or magnetic fields suggests that these rays don't carry an electric charge. Since they travel at the speed of light, they are classified as a form of electromagnetic radiation that carries even more energy than x-rays.



    الحمـــــــــــــــــــــد لله رب العالمــــــــــــــــين

  2. #2
    عضو ذهبي الصورة الرمزية زهراء غانم
    تاريخ التسجيل
    Jul 2011
    الدولة
    الاردن - ماركا
    المشاركات
    936

    افتراضي رد: Nuclear Chemistry

    Growth and Decay Curves
    At the turn of the century, when radioactivity was discovered, atoms were assumed to be indestructible. Ernest Rutherford and Frederick Soddy, however, found that radioactive substances became less active with time, as shown in the figure below. More importantly, they noticed that radioactivity was always accompanied by the formation of atoms of a different element. By 1903, they concluded that radioactivity was accompanied by a change in the structure of the atom. They therefore assumed that radiation was emitted when an element decayed into a different kind of atom.
    Growth and decay curves reported by Rutherford and Soddy for "uranium X" produced when uranium undergoes radioactive decay. Curve B shows decay in the activity after "uranium X" is extracted from uranium. Curve A shows growth in the activity of uranium as "uranium X" is replenished by radioactive decay.



    By 1910, 40 radioactive elements had been isolated that were associated with the process by which uranium metal decayed to lead. This created a problem, however, because there was space for only 11 elements between lead and uranium. In 1913, Kasimir Fajans and Fredick Soddy proposed an explanation for these results based on the following rules.

    • -particles are emitted when an element is formed that belongs two spaces to the left in the periodic table. Uranium (Z = 92), for example, emits an -particle when it decays to form thorium (Z = 90).
    • �/i>-particles are emitted when an element is formed that belongs one space to the right in the periodic table. Actinium (Z = 89) emits a �/i>-particle when it decays to form thorium (Z = 90).
    • Radioactive elements that fall in the same place in the periodic table are different forms of the same element. The radioactive thorium produced by the -particle decay of uranium is a different form of the element than the radioactive thorium obtained by the �/i>-particle decay of actinium.
    Soddy proposed the name isotope to describe different radioactive atoms that occupy the same position in the periodic table. J. J. Thomson and Francis Aston then used a mass spectrometer to show that isotopes are atoms of the same element that have different atomic masses.

    الحمـــــــــــــــــــــد لله رب العالمــــــــــــــــين

  3. #3
    عضو مميز الصورة الرمزية قمر بلحاج
    تاريخ التسجيل
    Jul 2012
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    5,302

    افتراضي رد: Nuclear Chemistry

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