P4: Atomic structure

• Atoms are the building blocks of all matter
• Atom


  Tiny, dense nucleus at the centre, with electrons orbiting around the nucleus
• The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom
• Parts of the Atom

  • Nucleus containing protons and neutrons
  • Electrons orbiting the nucleus
• Protons


  • Positively charged particles with a relative atomic mass of one unit
• Neutrons


  • No charge, and also with a relative atomic mass of one unit
• Electrons


  • Negative charge with almost no mass (1/2000 the mass of a proton or neutron)
• Energy levels


  Particular distances at which electrons orbit around the nucleus
• The total charge within an atom is zero because the number of electrons is equal to the number of protons
• Ionised


  When an atom loses electrons
• Electrons absorb electromagnetic radiation

  They move to a higher energy level
• Electrons move back down to their original energy level


  They emit a wave of electromagnetic radiation
• Atomic number


  The number of protons in an atom
• Mass number


  The total number of particles (protons and neutrons) in the nucleus of an atom
• Nuclear notation


  The mass number and atomic number of an atom shown with the atomic symbol
• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
• Some isotopes are unstable as they have an imbalance of protons and neutrons
• Positive ion


  An atom that has lost one or more electrons and is therefore positively charged
• Ions are more chemically reactive
• 0 neutrons
• Determine the number of electrons

  The number of electrons is equal to the number of protons
• Both chlorine-35 and chlorine-37 have 17 electrons
• Positive Ions


  Electrons in the outer energy level can be knocked out from an atom
• How positive ions form

  1. When objects are rubbed together, electrons can be removed by friction

  2. When electrons absorb electromagnetic radiation they can gain enough energy to leave the atom

  3. From chemical reactions
• Positive ion


  When one or more electrons are removed from an atom, it becomes positively charged
• Ion


  An atom or particle with a non-zero charge
• Ions are more chemically reactive than atoms because of their positive charge
• Plum Pudding Model


  The atom was thought to consist of negatively charged electrons (the 'plums') in a positively charged 'dough' or 'pudding'
• The Plum Pudding Model was replaced by the nuclear model as it could not explain the results of Rutherford's scattering experiment
• Nuclear Model


  • Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
  • The nucleus is positively charged
  • Negatively charged electrons orbit the nucleus at a distance
• Bohr's Model of the Atom

  • Electrons orbit the nucleus at different distances
  • The different orbit distances are called energy levels
  • Up to 2 electrons orbit in the first energy level
  • Up to 8 electrons can orbit in the second energy level
  • Up to 8 electrons can orbit in the third energy level
• The Bohr model became the accepted model because it could explain the findings from different experiments better than the nuclear model, it could explain the processes of absorption and emission of electromagnetic radiation, and theoretical calculations made using the Bohr model agreed with experimental results
• Proton


  The smallest positively charged particles in the nucleus
• Neutron


  A particle in the nucleus with no charge
• The discovery of the neutron allowed a better model of the atom to be created, with a positive, dense nucleus made from neutrons and protons, and negative electrons on different energy levels orbiting the nucleus
• Scientific models are used to explain observations. Models of the atom have changed and improved throughout history as new evidence became available
• Differences between Plum Pudding and Nuclear models

  • Plum Pudding: Atom consists of positively charged 'dough' with negatively charged 'plums'
  • Nuclear: Nearly all mass concentrated in positively charged nucleus, with negatively charged electrons orbiting
• Rutherford's alpha scattering experiment

  Led to the replacement of the Plum Pudding model with the Nuclear model
• The Plum Pudding model was replaced by the nuclear model