Transition elements

• Transition element: a d block element that forms at least one stable ion with a partially filled d subshell

  • exceptions: Sc and Zn (Sc3+ has no d electrons, Zn2+ has completely filled d subshell)
• atomic radius of elements in d block is smaller than s block:

  • nuclear charge increases, 3d electrons added are ineffective in shielding 4s electrons from nucleus (due to diffused 4-lobe shape of d orbitals)
  • effective nuclear charge increases
  • electrostatic forces of attraction between nucleus and valence electrons increases
• atomic radii of transition elements remain relatively constant, first IE remains relatively constant:

  • nuclear charge increases
  • electrons added to the inner 3d orbitals, shielding effect increases
  • increase in shielding effect almost cancels out increase in nuclear charge
  • effective nuclear charge increases gradually, electrostatic forces of attraction increase only slightly
  • energy needed to remove valence electrons increases only slightly
• transition elements are denser than s block elements:

  • transition metal ions have smaller atomic radii and higher atomic mass
  • more closely packed molecules due to stronger metallic bonding
  • more atoms can be packed in a unit volume
• d-d transition:

  • presence of ligands splits the d-orbitals into 2 different energy levels
  • electron in the lower energy d-orbital absorbs certain wavelengths of light energy from the visible region of the electromagnetic spectrum, is promoted to a higher energy d orbital
  • remaining wavelengths are transmitted, complementary colour is observed
• +2 oxidation state more stable compared to +3 oxidation state across the series
• M2+ and M3+ less stable compared to M(s)
• A complex: central metal atom/ion surrounded by other ions or molecules called ligands bonded to the central atom/ion by dative covalent bonds
• transition metals have:

  • high charge density, high polarising power to attract ligands
  • low-lying vacant d-orbitals that can accept lone pairs of electrons on ligands via dative covalent bonds
• ligand: neutral molecule or an anion with at least one atom bearing a lone pair of electrons to be used in dative covalent bond formation
• small energy gap, longer wavelength absorbed, remaining shorter wavelengths transmitted to show complementary colour
• large energy gap, shorter wavelengths absorbed, remaining longer wavelengths transmitted to show complementary colour
• weak field ligands result in smaller energy gap, longer wavelength of light absorbed, shorter wavelength transmitted
• strong field ligands result in larger energy gap, shorter wavelength of light absorbed, longer wavelength transmitted
• weak field ligands have weaker metal-ligand bonds, strong field ligands have stronger metal-ligand bonds
• transition metals and ions can act as heterogenous catalysts:

  • have partially filled 3d orbitals that allow for adsorption, d electrons used to form bonds with reactant molecules, energetically accessible and vacant orbitals used to accommodate lone pairs of electrons from reactant molEccles to form bonds
• adsorption: adhesion of a substance to the surface of another substance

  • weakens covalent bonds within reactant molecules, reduces activation energy needed for reaction
  • increases concentration of reaction molecules at catalyst surface, allows molecules to come into close contact with proper orientation for reaction
• homogenous catalyst:

  • catalyst and reactants in same physical state
  • provides an alternative pathway with lower activation energy by forming an intermediate which will be consumed in the reaction
  • transition metal ions can exist in different oxidation states and undergo conversion from one oxidation state to another easily via redox reactions