Which element has the highest first ionization energy in period 2?

First ionization energy generally increases across a period because nuclear charge grows and atomic radius shrinks, making it harder to remove an outer electron. In period 2: Li (520) < Be (899) ≈ B (801) < C (1086) < N (1402) > O (1314) < F (1681) < **Ne (2081)** kJ/mol. Neon has the highest because it has a stable octet ($1s^2 2s^2 2p^6$, full $2p$). Removing an electron requires breaking that closed shell. Note two anomalies: B < Be (removing a $2p$ electron easier than $2s$) and O < N (paired $2p$ electron in O has extra repulsion).