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Q.
Ionisation enthalpy increases across the second period as we move from $Li$ to $F$. This is because
Classification of Elements and Periodicity in Properties
Solution:
On moving from lithium to fluorine across the second period, successive electrons are added to orbitals in the same principal quantum level and the shielding from the nuclear charge by the inner core of electrons does not increase very much to compensate for the increased attraction of the electron to the nucleus.
Thus, across a period, increasing nuclear charge outweighs the shielding. Consequently, the outermost electrons are held more and more tightly and the ionisation increases across a period.