Which of the following atoms has the highest first ionization energy?
Sodium (Na)
Aluminum (Al)
Phosphorus (P)
Sulfur (S)
📝 Detailed explanation
Ionization energy generally increases across a period. However, Phosphorus (group 15) has a half-filled 3p³ subshell, which provides extra stability. Sulfur (group 16) has a 3p⁴ configuration where electron-electron repulsion in the first paired p-orbital makes it easier to remove an electron compared to P.
