First Ionization Energy
The energy needed to remove the single most loosely held electron from a neutral, gas-phase atom.
See first ionization energy colored on the interactive table →Across a period (left → right)
Rises as you move left to right across a period.
Down a group (top → bottom)
Falls as you move down a group.
Why it happens
This is really the same story as atomic radius, told from the opposite direction: a smaller atom holds its outer electrons closer to the nucleus and more tightly, so it takes more energy to pull one away. That's why ionization energy climbs across a period as atoms shrink, and drops down a group as the outermost electron gets steadily further from the nucleus (and increasingly shielded by the electron shells in between).
The rise across a period isn't perfectly smooth — a full or half-full subshell (like nitrogen's half-filled 2p³) is extra stable, so the very next element (oxygen) can actually have a slightly *lower* first ionization energy than expected.