How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?
First Ionisation Enthalpy: The irregular variation occurs because removal of one electron alters the relative energies of 4s and 3d orbitals. As we move along the 3d series, electrons are added to inner 3d orbitals while nuclear charge increases. These 3d electrons shield the 4s electrons more effectively from the nuclear charge, causing atomic radii to decrease less rapidly. Thus, ionization energies increase only slightly, creating an irregular pattern.
Second Ionisation Enthalpy: A general trend of increasing values is expected as effective nuclear charge increases, since one d electron does not shield another electron effectively. However, the steady increase breaks at Mn²⁺ formation because the ion achieves a stable d⁵ configuration, causing an anomaly in the otherwise increasing trend.
Explanation
The textbook explains that first ionisation enthalpy irregularity arises from the altered relative energies of 4s and 3d orbitals upon electron removal, combined with effective shielding by 3d electrons. For second ionisation enthalpy, the expected steady increase breaks when Mn²⁺ (d⁵ configuration) forms, as d⁵ configuration provides extra stability. This is a key concept in understanding transition element trends.