respectively. In both the cases, ions have d configuration. Similar breaks occur at corresponding elements in the later transition series. The interpretation of variation in ionisation enthalpy for an electronic configuration d n is as follows: The three terms responsible for the value of ionisation enthalpy are attraction of each electron towards nucleus, repulsion between the .
. Ionisation Enthalpies . In the series Sc ( Z = ) to Zn ( Z = ), the enthalpy of atomisation of zinc is the lowest, i.e., kJ mol – . Why?
electrons and the exchange energy. Exchange energy is responsible for the stabilisation of energy state. Exchange energy is approximately proportional to the total number of possible pairs of parallel spins in the degenerate orbitals. When several electrons occupy a set of degenerate orbitals, the lowest energy state corresponds to the maximum possible extent of single occupation of orbital and parallel spins (Hunds rule).
The loss of exchange energy increases the stability. As the stability increases, the ionisation becomes more difficult. There is no loss of exchange energy at d configuration. Mn + has d s configuration and configuration of Cr + is d , therefore, ionisation enthalpy of Mn + is lower than Cr + .
In the same way, Fe + has d configuration and Mn + has d configuration. Hence, ionisation enthalpy of Fe + is lower than the Mn + . In other words, we can say that the third ionisation enthalpy of Fe is lower than that of Mn. The lowest common oxidation state of these metals is + .
To form the M + ions from the gaseous atoms, the sum of the first and second ionisation enthalpy is required in addition to the enthalpy of atomisation. The dominant term is the second ionisation enthalpy which shows unusually high values for Cr and Cu where M + ions have the d and d configurations respectively. The value for Zn is correspondingly low as the ionisation causes the removal of one s electron