4.1 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 the protons
• Ways an atom's electron arrangement can change

  1. Absorbing electromagnetic radiation

  2. Emitting electromagnetic radiation
• Absorbing EM radiation


  Electrons move further away from nucleus to higher energy level
• Emitting EM radiation


  Electrons move closer to nucleus to lower energy level
• Atoms have no overall charge because the number of protons equals the number of electrons
• 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


  Lose one or more outer electrons, resulting in a positive charge
• Scientific models may be changed or replaced due to new experimental evidence that doesn't agree with the existing theory
• Plum-pudding model


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

  Rutherford's alpha-scattering experiment
• Currently accepted atomic model

  Bohr nuclear model
• Conclusions of the alpha-scattering experiment

  • Most of the mass is concentrated in the positively charged nucleus at the centre
• A scientific theory is reinforced when experimental results agree with the hypothesised theoretical calculations and theories
• James Chadwick's experiments proved the existence of neutrons