Topic 4 - Atomic Structure

• Plum Pudding Model


  A sphere of positive charge, with negatively charged electrons in it.

  
• The idea of the atom as the building block of matter has developed over time
• Alpha particle


  Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus)
• Vacuum


  A volume that contains no matter
• Rutherford's experiment


  1. Directed a beam of alpha particles at a thin gold leaf

  2. Observed the deflection of the alpha particles
• Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking
• It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it
• If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected
• Rutherford's observations


  • Most alpha particles passed straight through the foil
  • A small number of alpha particles were deflected by large angles (> 4°)
  • A very small number of alpha particles came straight back off the foil
• Nucleus


  The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.
• The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford
• The nucleus was calculated to be about 1/10,000th the size of the atom
• The idea of the atom as the building block of matter has developed over time
• What was thought of as a single particle about 1 × 10‾¹⁰ m across is now known to be a collection of smaller particles
• Atom


  • It is the building block of matter
• Developing the atom


  1. Rutherford and the nucleus

  2. Further developments to the atomic model
• Nucleus


  The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom
• Electron


  Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons
• Niels Bohr


  Revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus
• Bohr's 'solar system' model of the atom is the way that most people think about atoms today
• When atoms absorb energy

  1. Electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus)

  2. They jump back down to a lower level releasing light of definite frequencies
• Proton


  Nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass
• Neutron


  Neutral particle the same size as a proton to keep the nucleus stable and to make up the mass
• It wasn't until 1932 that James Chadwick was able to prove the existence of the neutron
• Chadwick's experiment

  1. Used a sheet of beryllium and a paraffin block instead of gold foil

  2. Able to prove that a proton-sized neutral particle - now known as the neutron - existed
• Nucleus


  The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.
• Neutron


  Uncharged subatomic particle, with a mass of 1 relative to a proton.
• Proton


  Subatomic particle with a positive charge and a relative mass of 1.
• Periodic table


  A tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table.
• Elements with fewer protons, such as the ones near the top of the periodic table, are stable if they have the same number of neutrons and protons.
• For example carbon, carbon-12 is stable and has six protons and six neutrons.
• As the number of protons increases, more neutrons are needed to keep the nucleus stable. For example lead, lead-206 has 82 protons and has 124 neutrons.
• Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay by emitting radiation.
• Radioactive decay


  The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so.
• Radiation


  Energy carried by particles from a radioactive substance, or spreading out from a source.
• Radioactive decay


  The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so
• Unstable nucleus


  • Can decay by emitting an alpha particle, a beta particle, a gamma ray or a single neutron
• Alpha particle


  A 'package' of two protons and two neutrons (the same as a helium nucleus)
• Alpha decay


  Causes the mass number of the nucleus to decrease by four and the atomic number to decrease by two
• Beta particle


  A fast-moving electron emitted when a neutron turns into a proton