F325 - OCR Chemistry
The transition elements occur in period 4 of the periodic table.
Transition element = a d-block element that forms a stable ion which has partially filled d-orbitals or an incomplete d-subshell.
The 4s subshell is at a lower energy level than the 3d subshell and therefore the 4s subshell fills before the 3d subshell. The orbitals in the 3d subshell are first occupied singly to prevent any repulsion caused by pairing. The majority of transition elements form ions in more than one oxidation state. When transition elements form ions they do so by losing electrons from the 4s orbitals before the 3d orbitals.
Sc and Zn each form ions in one oxidation state only: Sc3+ and Zn2+. The electron configurations of these ions are [Ar] and [Ar] 3d10 respectively. Neither fits the definition of a transition metal element.
However, there are two exceptions: Cu and Cr. Chromium has only one electron rather than two in the 4s sub-shell because this arrangement is more stable. Copper also only has one electron in its 4s sub-shell so that it has a full (and more stable) 3d sub-shell.
Properties of Transition Elements
- The transition elements are all metals and therefore they are good conductors of heat and electricity.
- They are denser than other metals. They have smaller atoms than the metals in groups 1 and 2 so the atoms pack more closely together, hence the density increases.
- They have higher melting and boiling points than other metals. Within the metallic lattice, ions are smaller than those of s-block metals which results in greater ‘free electron density’ and hence a stronger metallic bond.
- They form compounds with two or more oxidation states. This is because successive ionisation energies of transition metals increase only gradually. All the transition metals can form an ion of oxidation state 2+, representing the loss of two 4s electrons. The maximum oxidation state possible cannot exceed the total number of 4s and 3d electrons in the electron configuration.
- In their oxidation states they form coloured compounds. The colours are often distinctive and can be used as a means of identification.
- Many transition metals are used as heterogeneous catalysts – for example iron in the Haber process, nickel in the hydrogenation of alkenes… etc. They work as catalysts because their d-orbitals bind other molecules or ions to their…