First ionisation energy (IE1) = energy to remove the outermost electron from a gaseous atom. IE increases across a period (higher Z_eff) and decreases down a group (larger n). Exceptions: (1) Be (1s^2 2s^2) > B (1s^2 2s^2 2p^1) — removing p electron is easier than removing s electron due to less penetration. (2) N (2p^3, half-filled) > O (2p^4) — O has one paired 2p electron; electron-electron repulsion makes it easier to remove. These exceptions are directly related to subshell stability from Hund's rule. IE is related to Bohr model for hydrogen-like species: IE = 13.6 Z^2/n^2 eV (for removing the electron completely).