The ionisation enthalpy depend on the following factors: (i) Half-filled or fully filled atomic orbitals are more stable than other incomplete orbitals. (ii) Smaller the size of atom/ion, more is the ionisation energy. (I.E.). For 2nd I.E. all elements have M+ configuration
Total no. ofelectrons
Outermost configuration
Li+(Z=3)
2
1s2 (fully filled)
Be+(Z=4)
3
2s1 (has more size than carbon)
B+(Z=5)
4
2s2 (fully filled)
C+(Z=6)
5
2s22p1 (smaller size)
Hence, correct order for 2 nd I.E. is Li+>B+>C+>Be+ or Li>B>C>Be