Transitional elements

• Chromium has a unique electron configuration as an electron is taken from the 4s subshell and is held in the 3d subshell. This is because it creates a half full 4s subshell and a half full 3d subshell which is more stable.
• Copper like chromium has a unqiue electron configuration as an electron is taken from the 4s subshell and held in the 3d subshell to increase stability as it creates a fully 3d subshell.
• The 4s subshell is filled before 3d and 4s electrons are lost before 3d
• Scandium only forms 3+ ions. It has no 3d subshell.

  Zinc only forms 2+ ions. It has a full 3d subshell.

  Neither are transitional elements
• What are the general properties of transitional elements?

  • Variable oxidation states
  • Form complex Ions
  • Form coloured compounds
  • Act as catalysts
• In solution, transition elements form complex ions.

  A complex ion has a central transitional metal ion surrounded by ligands.
• A ligand is a molecule or ion that forms a dative covalent (coordinate) bond with a central transitional metal.
• An example of a complex ion is CuSO4 disolved in water. The Cu2+ ion becomes surrounded by 6 water molecules which form dative bonds with the copper ion.

  This is known as a Hexa-aqua copper 2+ ion
• When a transitional metal bonds with a ligand, the energy level of 3d electrons increase. This is because the ligands repel the 3d electrons. All of the electrons are not raised to the same energy level with some being raised higher than others.

  
  

  This is why transitional metals form colourful compounds, electrons on lower levels can be promoted if they aborb energy from a certain wave length of white light. Once the wavelength is absorbed the remaining light is released, reflecting a colour.
• An example of transitional metals acting as catalysts is in a catalytic converter which expresses heterogenous catalysis.

  2NO and 2CO bond with the catalyst surface, bringing the two reactants closer together (adsorption) and weakening the bonds within them, lowering the activation energy.

  Once reacted, the products leave the catalyst surface (desorption)
• An example of homogenous catalysis in transitional elements is the reaction between S2O82- and I- which is catalysed by Fe2+ or Fe3+.

  Fe2+ reacts with the S2O82- to form 2SO42- and Fe3+

  Fe3+ reacts with 2I- to form I2 and Fe2+

  The produts react more readily with one another and so the reaction occurs at a lower activation energy.
• Transitional element ions in solution react with other aqueous ions to form an insoluble precipitate.
• Cu2+(aq) + 2OH-(aq) --> Cu(OH)2(s)

  
  

  Pale blue precipitate

  
  

  [Cu(H2O)6]2+ + 2OH- --> [Cu(OH)2(H2O)4] + 2H2O
• Co2+(aq) + 2OH-(aq) --> Co(OH)2(s)

  blue precipitate

  [Co(H2O)6]2+ +2OH- --> [Co(OH)2(H2O)4] + 2H2O
• Fe2+(aq) + 2OH-(aq) --> Fe(OH)2 (s)

  green precipitate

  [Fe(H2O)6]2+ + 2OH- --> [Fe(OH)2(H2O)4] + 2H2O
• Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s)

  orange/brown precipitate

  [Fe(H2O)6]3+ + 3OH- --> [Fe(OH3)(H2O)3] + 3H2O