• All atoms consist of a small, massive nucleus surrounded by smaller, lighter electrons
  • the nucleus consists of protons and neutrons
  • the electrons are negatively charged (-q), protons are positively charged (+q), neutrons are neutral (zero charge)
    • Fundamental unit of electric charge: q = 1.6x10^-19 C

• The number of protons and electrons in an atom are equal; the net atomic charge is zero, therefore the atom is electrically neutral
• On the basis of the QUANTUM MODEL of the atom
  • electrons are held in stable ORBITS (ENERGY SHELLS) around nucleus by a balance between two opposing forces:
  • the force of electrostatic attraction pulls the negative electron towards the positive nucleus
    • known as the COULOMB ELECTROSTATIC FORCE
  • the force due to the mass-acceleration of the electron acts radially outwards from the nucleus
    • known as the CENTRIPETAL FORCE
      • this is the force which acts along a string when a stone is spun around your head
  • this results in a BINDING ENERGY which holds electrons in orbit around the nucleus
• The bound electrons do not possess any value of energy, but can only possess specific discrete energies according to the allowed orbits
  • the energy is said to be QUANTISED and the permitted values of energy are known as energy levels
  • the further the electron is from the nucleus the less tightly bound it will be to the nucleus.

• Each atom has an infinity of possible orbits (energy shells)
  • the electrons are confined to orbit in these discrete shells
  • not all shells will contain an electron
  • the number of electrons in a given shell is restricted to specific values
• The outermost occupied shell is known as the VALENCE SHELL
  • the valence electrons are very important
    • they determine material properties
  • the maximum number of valence electrons is 8
    • an atom with a valence of 8 is a stable atom (inert or unreactive)
    • an atom with a valence of 1 is highly reactive

Information provided by: http://vcs.abdn.ac.uk