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 (1$s^2$ 2$s^2$) > B (1$s^2$ 2$s^2$ 2$p^1$) — removing p electron is easier than removing s electron due to less penetration. (2) N (2$p^3$, half-filled) > O (2$p^4$) — 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).