General

• 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


  Number of protons in an atom
• Mass number


  Total number of protons and neutrons in an atom
• Isotope

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

  1. Lose one or more outer electrons

  2. Resultant charge is positive
• New experimental evidence that doesn't agree with existing theory can lead to a scientific model being changed or replaced
• Plum-pudding model

  Ball of positive charge with electrons distributed evenly throughout
• Prior to discovery of electron, atom was believed to be indivisible
• Experiment that led to discarding plum-pudding model


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

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