The second ionisation enthalpy refers to the energy required to remove a second electron from an ion. This value is highest for ions where the removal of the second electron would break a stable, fully-filled or half-filled electron configuration.
- For Na, after the first ionisation, the ion Na has a stable noble gas configuration (Ne), and it becomes very difficult to remove a second electron, resulting in a very high second ionisation enthalpy.
- For F, the second ionisation would remove an electron from a highly electronegative ion (F ), but it’s not as high as Na.
- For Ne, it already has a stable noble gas configuration and does not easily form a positive ion with a second ionisation.
- Mg also has a relatively high second ionisation enthalpy, but it’s not as high as that of Na, as Mg’s second ionisation takes it to a stable configuration.
Thus, the second ionisation enthalpy is highest for Na.