Unit 1.2

• Radioactive isotopes


  Unstable isotopes, usually heavy nuclei like those of uranium and plutonium
• Radioactive disintegration


  1. Spontaneous disintegration of radioactive isotope nuclei

  2. Emission of alpha particles (helium nuclei)

  3. Emission of beta (electron) particles

  4. Emission of gamma (high energy electromagnetic) rays
• Half-life


  Time taken for radioactivity to fall to half of its initial value, independent of radioisotope mass
• Half-life is characteristic of each radioisotope and unaffected by catalysts or changes in temperature
• Types of radioactive emissions


  • Alpha (helium nuclei)
  • Beta (electrons)
  • Gamma (high energy electromagnetic radiation)
• Alpha radiation


  • Positive, heavy, slow moving, slightly attracted to negative plate in electric field
• Beta radiation


  • Light, fast moving, considerable deviation towards positive plate in electric field
• Gamma radiation


  • Electromagnetic radiation of short wavelength, unaffected by electric field
• Alpha and beta radioactive emissions result in formation of a new nucleus with different atomic number, a different element
• Gamma radiation reduces the energy of the nucleus
• Alpha emission


  • Protactinium-231 decays to thorium-227
• Beta emission


  • Lead-131 decays to bismuth-131
• A neutron in the nucleus changes to a proton and an electron during beta emission
• Energy levels


  Electrons occupy certain fixed energy levels in an atom, with level 1 closest to nucleus and level infinity corresponding to no attraction to nucleus
• Orbitals


  Areas of space with high probability of finding an electron, can have different shapes like spherical s-orbitals
• First ionisation energy


  Energy required to remove one electron from each of a mole of free gaseous atoms
• First ionisation energy depends on number of protons, shielding by inner electrons, effective nuclear charge, and electron repulsion
• Across a period in periodic table


  First ionisation energy increases due to increasing nuclear charge but constant shielding
• Comparing first ionisation energies of elements


  Depends on factors like nuclear charge, shielding, effective nuclear charge, and electron repulsion
• Helium has the highest first ionisation energy of all elements
• Emission spectrum


  Series of sharp distinct lines formed when light from a flame test source is examined by a prism, due to electronic transitions
• Absorption spectrum


  Series of dark lines against a bright background, formed when white light passes through gaseous atoms and photons of certain energies are absorbed
• Helium was discovered by its absorption spectrum in solar radiation before being isolated on Earth