The Pauli Exclusion Principle

The Chemistry of Art > 3. Electrons in the Atom > The Pauli Exclusion Principle >

Define the Pauli exclusion principle to identify the position of electrons around an atom

Placement of electrons:

    • Maximum number of electrons a principal energy level can hold = 2n2
    • Every shell apart from n=1 has a number of energy sublevels or sub-shells with slightly different energies
    • Number of subshells in a shell = To principal quantum number of the shell

e.g. n=3 has three subshells

    • Unlike Bohr model, quantum mechanical model shows precise location of electron with specific energy cannot be known

Instead linked to probability

  • Region of space through which an electron of given energy may move = Orbital
  • Each Orbital can hold a maximum of two electrons


Sub Shells:

    • Lowest subshell = s sub shell
      • One orbital
      • Hence two electrons
      • Distribution spherically symmetrical
    • Second lowest subshell = p sub shell
      • Three identical orbitals
      • Hence six electrons
      • Identical in shape and oriented in right angles to each other
    • Third lowest subshell = d sub shell
      • Five orbitals
      • Hence 10 electrons
    • Fourth slowest subshell = f sub shell
      • Seven orbitals
      • Hence fourteen electrons
    • To specify shell and subshell of electron
        • Write down principal quantum number “n”
        • Follow this with energy subshell ‘s < p< d <f ‘

      e.g. 3d = Electron in d subshell of third energy level

    • Remember: Within any subshell, the electron exist in orbitals

Regions of space where an electron is likely to be located


    • In certain respects, elctrons behave as if they are spinning on an axis

They can therefore spin in either a clockwise or anticlockwise direction

  • Spinning charges produce magnetic fields
  • Two opposite spins result in splitting of spectral lines


Pauli Exclusion Principle:
“an orbital can hold a maximum of two electrons that must have opposite spins”

Hund’s Rule
“If two or more orbitals with the same energy (in the same subshell) are available, one electron goes into each orbital until all are half full, and only then does a second electron fill an orbital”