Atoms and Isotopes

• Atom radius


  1x10⁻¹⁰ metres
• Subatomic constituents of an atom

  • Proton
  • Neutron
  • Electron
• Most of the mass of an atom is concentrated in the nucleus
• Arrangement of protons, neutrons and electrons in an atom

  1. Protons and neutrons in the nucleus

  2. Electrons in discrete energy levels around the nucleus
• Charge of the nucleus

  Positive charge, due to protons
• Ways an atom's electron arrangement can change

  1. Absorbing electromagnetic radiation

  2. Emitting electromagnetic radiation
• How electron arrangement changes when atom absorbs EM radiation

  1. Electrons move further away from nucleus

  2. Electrons move to higher energy level
• How electron arrangement changes when atom emits EM radiation

  1. Electrons move closer to nucleus

  2. Electrons move to lower energy level
• Reason atoms have no overall charge

  Number of protons equals number of electrons, so charges cancel
• All forms of the same element have the same number of protons
• Atomic number


  The number of protons in an atom
• Mass number


  The total number of protons and neutrons in an atom
• Isotope


  An atom of an element with a different number of neutrons but the same number of protons
• How atoms turn into positive ions


  They lose one or more of their outer electrons, resulting in a positive charge
• New experimental evidence that doesn't agree with existing theory may lead to a scientific model being changed or replaced
• Plum-pudding model


  A ball of positive charge with negatively charged electrons distributed evenly throughout it
• Prior to the discovery of the electron, the atom was believed to be indivisible
• Experiment that led to discarding the plum-pudding model


  Rutherford's alpha-scattering experiment
• Currently accepted model of the atom

  The Bohr nuclear model
• Conclusions of the alpha-scattering experiment
• Experimental results agreeing with theoretical calculations and theories reinforces a scientific theory
• James Chadwick's experiments proved the existence of neutrons