Transition Elements

• 
  

  A transition element is a d block element that forms 1 or 2 stable ions with an incomplete d electron sub shell. They are period 4
• Sc and Zn


  The are not transition metals as Sc does not have electrons in its d orbital and Zn d orbital is completed
• The range of oxidation states; common-()


  • Ti 1 2 (3) (4)
  • V 1 (2) (3) (4) (5)
  • Cr 1 2 (3) 4 5 (6)
  • Mn 1 (2) 3 (4) 5 (6) (7)
  • Fe 1 (2) (3) 4 5 6
  • Co 1 (2) (3) 4 5
  • Ni 1 (2) 3 4
  • Cu (1) (2) 3
• Transition elements may have several different oxidation states
• The most common oxidation state of transition elements is +2
• The maximum oxidation state of the transition elements up to Mn involves all the 4s and 3d electrons
• From Fe onwards, the +2 oxidation state dominates because the 3d electrons become increasingly harder to remove as the nuclear charge increases
• Higher oxidation states of transition elements are found in complex ions or compound ions such as MnO4- and Cr042-
• Characteristics of transition elements


  • They form compounds with different oxidation states
  • Transition element ions form coloured compounds
  • Transition element ions form complex ions
  • Transition elements and their compounds are often good catalysts
  • Transition elements have very high density
  • Transition elements have very high melting points and boiling points
• Atomic radius


  The decrease in atomic radii from left to right across the first row of the d-block is small and irregular. In the first row of the d-block, as the nuclear charge increases across the period, each additional electron enters the penultimate 3d orbital which increases the shielding experienced by the 4s electrons. This results in a relatively small difference in the effective nuclear charge as the shielding effect nullifies, to a large extent, the increase in nuclear charge. As the atomic radius decreases with increasing relative atomic mass, the density increases.
• Ionization energy


  The first ionization energy increases from left to right across the first row of the d-block. In the first row of the d-block, as the nuclear charge increases each additional electron enters the penultimate 3d orbital. These 3d electrons efficiently shields the 4s electrons from the nucleus. Thus the increase nuclear attraction for the outer 4s electrons is minimal across the period. This results in relatively small changes in the energy required to remove an outer 4s electron.
• Melting and boiling points


  The d-block elements form metallic structures. Electrons are available from both 4s and 3d orbitals for delocalization to form metallic bonds. This results in relatively high melting points and boiling points. The melting and boiling points drop at Mn (half filled) and Zn (filled). The d orbitals make the electrons less available for metallic bonding.
• Calcium and transition elements of Period 4

  Both are metals, but there are some marked differences between them
• Transition elements


  • Harder and have higher melting points than calcium
  • Stronger metallic bonding compared with calcium
  • Can release electrons from both the 4s and 3d sub-levels
• Metallic bonding in transition elements vs calcium


  Greater force of attraction between the small (and often highly charged) transition element ions and the sea of electrons than calcium
• Atomic and ionic radi of calcium


  Much larger than the corresponding radia for typical transition elements
• transition elements vs calcium

  Electrons in the d sub-shell are less good at shielding the outer electrons from the nuclear charge than s or p electrons
• First ionisation energies of transition elements vs calcium

  Higher than that of calcium due to the relatively greater force of attraction between the nucleus and the outer electrons
• Transition elements vs calcium


  Denser than calcium due to the fact that transition element ions have smaller radi than calcium
• Electronegativity of transition elements vs Calcium

  • Significantly higher compared with calcium
  • Increases across the series from Ti to Cu as the elements get slightly less metallic in character
• VO 3- + 2H+ -> VO₂+ + H₂O
• The colour of the solution changes of Vanadium is. from yellow (VO2. +) then to blue (VO 2+) then to green (V 3+) and finally to violet (V2+)
• Ligands

  Water molecules that form co-ordinate (dative covalent) bonds with the transition element ion
• Complex ion

  The ion formed when a transition element ion is bonded to ligands
• Degenerate orbitals

  3 d orbitals in an isolated transition element ion that all have the same average energy
• orbitals close to the ligand are pushed to slightly higher energy levels than those farther away. The orbitals split into two groups. When an electron moves from a D orbital of lower energy to orbital higher energy, light is absorbed in the visible region of the spectrum. The frequency of light absorb depends on the energy difference between the split the levels. Different ligands split the D energy levels by a different amount. So different ligands different colors to be absorbed.
• Ligand

  A molecule or ion with one or more lone pairs of electrons available to donate to a transition element ion
• Simple ligands

  • water
  • ammonia
  • chloride ions
• Co-ordinate (dative covalent) bonds

  Formed with the transition element ions because the d electrons in transition elements do not shield the outer electrons very well from the nuclear charge
• Complex ion

  Formed when the relatively poorly shielded nucleus attracts lone pairs of electrons strongly enough to form co-ordinate bonds
• Charge on a complex ion

  The sum of the charges on the transition metal ion and all the ligands
• Co-ordination number

  The number of co-ordinate bonds a ligand forms with the central transition metal ion
• Monodentate ligands

  • Form one bond per ligand
• Bidentate ligands

  • Form two bonds per ligand, have two lone pairs available to form co-ordinate bonds
• Hexadentate ligands

  • Form 6 bonds per ligand
• Ligand exchange


  If there is more than one ligand in a solution, they can compete for a transition metal cation
• Stability constant (K)


  • Equilibrium constant for ligand exchange reaction
  • Larger value = more stable complex
  • More likely complex will form
• Ammonia has higher stability constant than Cl-


  • Addition of ammonia shifts equilibrium to the right
  • Deep blue complex ion formed
• Haem


  • Red blood pigment
  • Found in haemoglobin
  • Fe2+ ion with coordination number 6
  • 4 nitrogen atoms in complex ring
  • 5th position with nitrogen in protein
  • 6th position with oxygen molecule
• Carbon monoxide binds to Fe2+ ion


  • Stability constant 200 times higher than O2
  • Hardly any oxygen will bind
  • Respiration inhibited
  • Often results in death