atomic structure

• Atomic theory


  The idea that everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Democritus proposed atomic theory


  Around 500 BC
• Atoms (according to Dalton)


  Solid spheres, different types of spheres make up different elements
• Plum pudding model (J.J. Thomson)


  Atom is a general ball of positive charge with discrete electrons stuck in it
• Rutherford's gold foil experiment


  1. Fired positively charged alpha particles at a thin sheet of gold

  2. Some alpha particles were deflected to the side or back, proving the plum pudding model wrong
• Rutherford's nuclear model


  Atom has a compact nucleus containing positive charge, with negative charge in a cloud around the nucleus
• Rutherford's nuclear model


  Had a flaw - the negative electrons should collapse into the positive nucleus
• Bohr's model


  Electrons orbit the nucleus in shells, preventing the atom from collapsing
• Further experiments by Rutherford found the positive charge in the nucleus is made up of protons
• James Chadwick provided evidence for neutral particles (neutrons) in the nucleus
• Atom


  The fundamental unit of an element, consisting of a nucleus containing protons and neutrons, and electrons that orbit the nucleus
• Nucleus


  • Contains protons and neutrons
  • Protons are positively charged
  • Neutrons are neutral
  • Protons and neutrons have a relative mass of 1
• Electrons


  • Orbit the nucleus
  • About 2000 times smaller than protons or neutrons
  • Have a negative charge of 1-
• Periodic table


  A tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties
• Nucleus symbol


  A box on the periodic table that represents a particular element
• Nucleus symbol


  • Contains the elemental symbol
  • Atomic number (number of protons)
  • Mass number (total protons and neutrons)
• The number of protons and electrons in an atom is the same
• Isotopes


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Generally only one or two of an element's isotopes are stable, the others are unstable and undergo radioactive decay
• Electrons


  • Arranged in a series of energy levels (shells)
  • Shells get progressively further from the nucleus and increase in energy level
  • Electrons can jump to higher energy levels if they gain enough energy (become excited)
  • Electrons will then fall back down and re-emit the energy as electromagnetic radiation
• Ionization


  When an outermost electron absorbs enough energy to completely leave the atom, leaving the atom with a positive charge (a positive ion)
• Ionizing radiation is able to knock electrons off atoms and ionize them
• Isotopes


  Different forms of an element, with the same number of protons but different numbers of neutrons
• Only one or two of an element's isotopes are stable, while the rest are unstable and undergo radioactive decay</b>
• Radioactive


  Consisting of unstable isotopes that can decay
• Types of nuclear radiation


  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Alpha particles


  • Made up of two protons and two neutrons, like the nucleus of a helium atom
  • Don't have any electrons, so have an overall charge of two plus
  • Easily stopped by collisions with other molecules, can only travel a few centimeters in air and are absorbed by a single sheet of paper
  • Strongly ionizing, can easily knock electrons off any atoms they collide with
• Beta particles


  • Just electrons, with a charge of -1 and virtually no mass
  • Emitted when a neutron in an atom decays into a proton and an electron
  • Moderately ionizing and penetrate moderately far into materials, stopped by several meters of air or about five millimeters of aluminium
• Gamma rays


  • Waves of electromagnetic radiation, often emitted after alpha or beta radiation
  • Don't have any mass or charge, tend to pass straight through materials rather than colliding with atoms
  • Weakly ionizing, can penetrate really far into materials before being stopped, require thick sheets of lead or multiple meters of concrete to stop them
• Neutron emission


  • Occurs when a nucleus contains too many neutrons, making it unstable, and it throws out a neutron to increase stability
• Nuclear equations for alpha, beta, and gamma radiation


  1. Alpha particles consist of two protons and two neutrons

  2. Unstable nucleus undergoes alpha decay and emits alpha particle

  3. Subtract 4 from mass number and 2 from atomic number

  4. Resulting element is different, check periodic table

  5. Add emitted helium
• Beta decay


  1. Neutron turns into proton and emits fast moving electron (beta particle)

  2. Atomic number increases by 1, mass number stays the same

  3. Represent beta particle with beta symbol and charge of -1
• Gamma radiation


  Pure energy, no mass or charge, doesn't change anything
• Neutron emission


  Subtract 1 from mass number to account for lost neutron
• Activity


  The overall rate of decay of all the isotopes in a sample, measured in becquerels (decays per second)
• Half-life


  The time taken for the number of radioactive nuclei in a sample to half, or for the activity to half
• Radioactive decay


  1. Unstable isotope emits radiation (alpha, beta, gamma) to become more stable

  2. Decay process is random, cannot predict when individual isotope will decay

  3. With a large sample, overall rate of decay (activity) can be measured

  4. As sample decays, number of unstable nuclei and activity decrease
• As number of radioactive nuclei decreases


  The rate of decay (activity) also decreases
• Geiger-Muller tube


  • Measures the count rate of radiation, which is used to estimate the activity
• Radioactive samples


  • Sample 1 with half-life of 2 hours
  • Sample 2 with half-life of 1 hour