Topic 2: Atomic Structure

• Nuclear symbol notation Z A X


  • Used to deduce the number of protons, neutrons, and electrons in atoms and ions
• Nucleus


  The central and most important part of an atom
• Atoms consist of three types of subatomic particle: proton, neutron, electron
• Atomic mass unit (amu)

  Convenient unit for the very small masses of subatomic particles, 1 amu = 1.660539 × 10^-24 g
• Subatomic particles


  • Proton
  • Neutron
  • Electron
• The neutron was discovered by British physicist James Chadwick in 1932
• Chadwick's experiment to discover the neutron

  1. Beryllium placed in vacuum chamber

  2. Bombarded with alpha particles from polonium

  3. Beryllium emitted neutrons
• Atomic number (Z)


  The number of protons in the nucleus of an atom of an element
• For a neutral atom, the number of electrons equals the number of protons
• Mass number (A)


  The number of protons + the number of neutrons in the nucleus of an atom
• Nuclear symbol


  Represents the element X with mass number A and atomic number Z as A/ZX
• Isotopes


  Different forms of the same element that have the same atomic number Z but different mass numbers A
• Isotopes of hydrogen


  • Protium (1/1H)
  • Deuterium (1/2H)
  • Tritium (1/3H)
• Most elements exist as mixtures of isotopes in nature
• Radioisotopes


  Radioactive isotopes
• Radioisotopes of boron


  • Boron-8
  • Boron-9
  • Boron-12
  • Boron-13
• Uses of radioisotopes


  Nuclear medicine for diagnostics, treatment, and research<|>Tracers in biochemical and pharmaceutical research<|>Chemical clocks in geological and archaeological dating
• Iodine-131


  Emits gamma rays, used to treat thyroid cancer and determine thyroid gland function
• Iodine-125


  Used to treat prostate cancer and brain tumours
• Positron emission tomography (PET) scanners


  Give 3D images of tracer concentration in the body, used to detect cancers
• Single-photon emission computed tomography (SPECT) imaging


  Used to detect gamma rays emitted from iodine-131
• Cobalt-60


  Emits gamma rays, used in radiotherapy to treat cancer
• Formation of carbon-14 in the atmosphere

  1. Nitrogen-14 in atmosphere

  2. Bombarded by cosmic rays

  3. Produces carbon-14 and hydrogen
• Half-life


  Time for radioactive isotope to decrease to half its initial value
• The half-life of carbon-14 is 5730 years
• Carbon-14 dating


  1. Living organisms absorb carbon-14 from atmosphere

  2. Ratio of carbon-12 to carbon-14 remains constant

  3. When organism dies, carbon-14 decays over time
• Nitrogen gas


  Consists of 78% of the Earth's air by volume
• Half-life (t1/2)


  The time it takes for an amount of radioactive isotope to decrease to one-half of its initial value
• The half-life for the carbon-14 decay process is 5730 years
• Carbon-14 decay


  1. Carbon-14 can be oxidized to form carbon dioxide

  2. Living plants absorb carbon dioxide for photosynthesis and assimilate the carbon into other compounds in their bodies

  3. Animals consume plants, taking in their carbon compounds, and they exhale carbon dioxide

  4. In all living organisms the ratio between carbon-12 and carbon-14 found in the atmosphere is essentially constant at any given time, since carbon is continually exchanged with the atmosphere in the processes of life

  5. When a living organism dies, its carbon is no longer exchanged with the atmosphere or with other organisms
• Carbon-14 decay


  Carbon-14 isotope may then undergo decay to form nitrogen, emitting beta particles (electrons) in the process
• The net result is that there is a gradual decrease in the ratio of carbon-14 to carbon-12 in the organism's body
• The amount of carbon-14 in the body of a plant or animal that was once living can be measured
• Scientists can use this method to determine the age of artefacts such as wood, paintings, papyrus, ancient manuscripts, and scrolls
• Relative atomic mass


  The mass of the electron is negligible (1/1836 amu). The mass of the atom is concentrated in the nucleus in the protons and neutrons
• Unified atomic mass unit

  A non-SI unit of mass and is defined as one-twelfth of the mass of a carbon-12 atom in its ground-state
• Relative atomic mass (Ar)


  The ratio of the average mass of the atom to the unified atomic mass unit
• The average mass of the atom is a weighted average of the atomic masses of its isotopes and their relative abundances
• Mass spectrometer


  An instrument used to determine the relative atomic mass of an element and show its isotopic composition
• Mass spectrometer operation


  1. Vaporization of sample

  2. Ionization of gaseous atoms

  3. Acceleration of positive ions

  4. Deflection of positive ions by magnetic field

  5. Detection of species with particular mass-to-charge ratio