Electronic structure

• Bohr model


  Early model of the atom with electrons in spherical orbits
• Bohr model


  • 2 electrons in first shell, 8 in second etc.
  • Atoms and ions with noble gas electron arrangements should be stable
• 1. level model


  Electrons are arranged on principle energy levels numbered 1,2,3,4.. and sub energy levels labelled s, p, d and f
• Electron sub-levels


  • s holds up to 2 electrons
  • p holds up to 6 electrons
  • d holds up to 10 electrons
  • f holds up to 14 electrons
• Orbitals


  Represent the mathematical probabilities of finding an electron at any point within certain spatial distributions around the nucleus
• Orbitals


  • Each orbital has its own approximate, three dimensional shape
  • It is not possible to draw the shape of orbitals precisely
• Principle energy levels


  • 1
  • 2
  • 3
  • 4
• Sub-levels


  • 1s
  • 2s, 2p
  • 3s, 3p, 3d
  • 4s, 4p, 4d, 4f
• Atoms fill up the sub shells in order of increasing energy (note 3d is higher in energy than 4s and so gets filled after the 4s)
• Spin diagrams


  An arrow is one electron, arrows going in opposite directions represent different spins
• 1. block element


  One whose outer electron is filling a s-sub shell
• Positive ion


  Formed when electrons are lost (e.g. Mg2+ is 1s2 2s2 2p6)
• Negative ion


  Formed when electrons are gained (e.g. O2- is 1s2 2s2 2p6)
• Electron configuration


  How electrons are arranged in an atom, in shells and subshells
• Subshells


  • s
  • p
  • d
  • f
• s subshell


  • Has 1 orbital that can hold 2 electrons
• p subshell


  • Has 3 orbitals that can each hold 2 electrons, for a total of 6 electrons
• d subshell


  • Has 5 orbitals that can each hold 2 electrons, for a total of 10 electrons
• f subshell


  • Has 7 orbitals that can each hold 2 electrons, for a total of 14 electrons
• Shells are made up of one s subshell and one p subshell
• Principal quantum number


  The shell number
• s orbital


  • Spherical shape that can hold 2 electrons
• p orbitals


  • 3 dumbbell-shaped orbitals at 90 degrees to each other, each can hold 2 electrons
• Electrons in the same orbital spin in opposite directions
• Electron configuration of iron (Fe)

  • 1s2 2s2 2p6 3s2 3p6 3d8 4s2
• Filling electron orbitals


  1. Fill lowest energy levels first

  2. Fill singly first, then pair up

  3. Electrons repel each other so occupy separate orbitals if possible
• Ion


  Atom that has lost or gained electrons
• Electron configuration of calcium 2+ ion (Ca2+)


  • 1s2 2s2 2p6 3s2 3p6
• Noble gas notation


  Using the symbol of a noble gas to represent the core electron configuration, then adding the remaining electrons
• Noble gas notation is a shorthand way to write electron configurations, but the full configuration may be required in some questions