Among the second period elements the actual ionization enthalpies are in the increasing order Lithium Boron beryllium carbon oxygen nitrogen fluorine and neon. Then explain why beryllium has higher ionization enthalpy than Boron and oxygen has lower ionization enthalpy than nitrogen and fluorine

• In case of Be electron has to be removed from an s-orbital whereas in Boron removal takes from p-orbital.
  

As s-orbital is closer to nucleus, hence removal of electron is difficult which raises its ionization enthalpy.

• Comparing O and N, nitrogen has completely half filled p-orbitals, hence I.E. of nitrogen is greater than oxygen. [Half filled orbitals account for greater stability].
  Comparing O and F, F has increased nuclear charge and small size, hence removal of electron is difficult. Therefore, I.E. of F is greater than O.