Question

why the firs ionisation enthalpy of flourine is greater than second

Answer 1

Firstly, ionisation energy is the amount of energy required to remove an electron from the valence shell in the gaseous state of an atom. As we move across the period, electrons are added to the same shell. Hence, effective nuclear charge increases. This translates to the fact that the electrons are held more tightly on moving across the period. Hence, fluorine has more I.E compared to oxygen. The fact that on losing an electron oxygen will attain the half filled configuration is more than compensated by the high electronegativity of fluorine. Hence, the following factors may be considered:
1) The increase in ENC across the period.
2) The increase in electronegativity across the period.
3) Half filled configuration attained by oxygen.
1) and 2) overpower 3) and hence fluorine has higher I.E than oxygen