The history of the atomic chronicle must also induce another great discovery, namely, the Phenomenon of Radioactivity.
    In 1896 the French scientist Becquerel, while investigating the nature of the mysterious X -rays discovered by Rontgen a few months earlier, found that a photographic plate wrapped in thick black paper was affected by a sample of potassium - uranyl - sulfate placed over it. In fact, any uranium compound would be effective the plate through covered by paper and kept away from light.
    The oblivious conclusion that some radiations emanating from the uranium compound could penetrate through the cover and attack the photographic plate.
    This penetrating radiation had its source in uranium itself and Becquerel christened this amazing behavior as Radioactivity. The properties of these radiations were very similar to those of X -rays.
  1. They were highly penetrating, they affected photographic plates, they would ionize gases and would also induce the fluorescence in some substances.
  2. The rays are not influenced by heat, light or chemical composition.

Discovery of radioactive elements

    Marie Curie found that the activity of mineral pitchblende was far greater than what was expected of its uranium content. In 1898 Pierre and Marie Curie actually isolated two new elements Polonium and Radium which were more radioactive compared to uranium, the heaviest atom known at the time.
    In 1900, Debierne and Giesel discovered actinium which was also radioactive. That the radioactive effects were essentially atomic was recognized early and this helps the isolation to a considerable extent.
    It was immaterial how uranium and radium chemically combined. The same number of radium atoms will always have the same activity independent of the physical state or environmental conditions.
    The phenomenon of radioactivity is associated with atoms that are heavier than lead or bismuth.
    The phenomenon of emission of radiation as a result of spontaneous disintegration in atomic nuclei was termed as radioactivity.

Radiation from radioactive elements

    The radiations emitted by naturally radioactive elements were shown to split by an electric or magnetic field into three distinct parts: Alpha(α), Beta(β) and Gamma(ɣ) Rays.

Alpha rays

    These consist of a stream of positively charged particle which carries +2 charge and has mass number is 4.
    These particles are shown by Rutherford to be identical with, the nuclei of the helium atom, that is, these are doubly charged helium ion He⁺²(atomic number 2, mass number 4).
    When an Alpha particle ejected from within the nucleus the mother element loss two units of atomic number and four units of mass number.
    92U238 92U234 + 2He4

Beta rays

    These are made up of a stream of negatively charged particles (beta particles). They have been shown to be identical with electrons from a study of their behavior in electric and magnetic fields and from the study of their e/m values (1.77 × 108 coulomb/gm). 
    The ejection of a beta particle (charge -1, mass 0) results from the transformation of a neutron (mass 1, charge 0) somewhere at the surface of the nucleus into a proton (mass 1, Charge +1). 
    0n11H1 + -1e0
    When a beta particle is emitted from the nucleus, the daughter element nucleus has an atomic number one unit greater than that of the mother element nucleus.
    90Th234  91Pa234 + -1e0
    Although beta particles and electrons are identical in their electrical nature and charge/mass ratio, there is a fundamental difference between them.
    The ejection of an electron from an atom converts a neutral atom into a positively charged ion but leaves the nucleus undisturbed. The ejection of a beta particle changes the very composition of the nucleus and produces an atom of the next higher atomic number.

Gamma rays

    These consist of electromagnetic radiation of very short wavelength (λ ∼ 0.005 - 1 Å). These are high energy photons.
    The emission of gamma rays accompanies all nuclear reactions. During all nuclear reactions there occurs a change in the energy of the nucleus due to the emission of alpha or beta particles. The unstable, excited nucleus resulting from the emission of an alpha or beta particle gives off a photon and drops a lower and more stable energy state.
    Gamma rays do not carry charge or mass, and hence emission of these rays cannot change the mass number or atomic of the mother nucleus.


    Since the works of the Curies and Rutherford yet another mode of nuclear transformation has been discovered. This involves the ejection of a positron ₊₁e⁰ from within the nucleus.
    This ejection is made possible by the conversion of a proton into a neutron.
    ₁H¹ → ₀n¹ + ₊₁e⁰ 
    The ejection of positron lowers the atomic number one unit but leaves the mass number unchanged. 
    ₅₁Sb¹²⁰ → ₅₀Sn¹²⁰ + ₊₁e⁰


    Breaking down of a neutron into a proton and a beta particle creates a problem with the principle of conservation of angular momentum. Particles like Neutron, Proton, and Electron have the spin angular momentum of ±1/2 (h/2π) each. It is thus seen that the equation:
    ₀n¹ ₁H¹ + ₋₁e⁰
    This is not balanced in so far as angular momentum is concerned. If the angular momentum of the proton and the electron are +1/2 (h/2π) they exceed the angular momentum of the neutron.
    If they oppose each other then the momentum becomes zero in violation of that of the neutron. Pauli, therefore, postulated that along with the ejected beta particle another tiny neutral particle called neutrino is also ejected.
    This neutrino has also spin angular momentum of ±1/2 (h/2π). The sum of angular momentum of the particles ejected {say +1/2 (h/2π) for proton, -1/2 (h/2π) for the electron and +1/2 (h/2π) for the neutrino} may now be +1/2 (h/2π) being the same as that of the neutron.
    The mass of the neutrino is around 0.00002 with respect to the oxygen scale. Ejection of an electron from within the nucleus should be represented as:
Radioactivity and artificial transmutation reactions
Radioactive reaction

Alpha, beta and gamma rays

 Radioactivity and properties of alpha beta and gamma rays
Alpha, beta and gamma rays

Nuclear and chemical reaction

Nuclear reactions are different from chemical reactions in many respects:
  1. Chemical reactions involve some loss, gain or overlap of outer orbital electrons of the reactant atoms. Such reactions cannot alter the composition of the nuclei so that the atomic number of the chemical reactions unchanged.
    CH4 + H2O CO + 3H2
    On the other hand cause of nuclear decay involves emission of alpha particles, beta particles or positrons from inside the nucleus, which leads to change in the atomic number of the nucleus.
    51Sb120  50Sn120 + +1e0
    In some artificially induced radioactive decay reactions, neutrons are absorbed by target nucleus producing isotopes. Nuclear reactions, therefore, leads either to the birth of another element or produce isotopes of the parent element.
  2. The nuclear reactions are accompanied by energy changes which far exceed the energy changes in chemical reactions.
    For example, the energy evolved in the radioactive transformation of one gram of radium is five hundred thousand times as large as the energy released when one gram of radium combine with chlorine to form RaCl2.

Radioactivity, Definition, Discovery and Characteristic of Alpha rays, Beta rays, Gama rays, Difference between Nuclear and Chemical reaction.

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