A transition element is a metal which forms one or more stable ions which have incompletely-filled d-orbitals.
- have variable oxidation states
- are good catalysts
- form coloured compounds (precipitates)
Successive ionisation energy is the energy required to remove, in turn, one electron from each ion in one mole of gaseous ions, under standard conditions.
In a transition element, electrons are initially removed from the 4s sub-shell, followed by the 3d sub-shell. There is only a very small energy difference between these two sub-shells.
When both sub-shells are empty, the 4s shell is filled first because it is slightly lower in energy.
However, 4s electrons are removed before the 3d electrons. This means that nearly all transitions metals form stable 2+ ions.
Transition elements can make available a variable number of d-electrons for covalent bonding (hence variable oxidation states). This also affects the number of ligands a metal ion can accept (see heading on complex ions).
The endothermic process of promoting electrons from a 3d sub-shell to a higher sub-shell is compensated by the energy released on the…