ionization energy
Imagine prising a magnet off a fridge door. Some magnets pop off with a flick; others are gripped so tight you have to wrench them. The energy it takes to pull an electron clean off an atom is just like that pull — and how hard it is reveals how firmly the atom holds onto its electrons.
Ionisation energy is the minimum energy needed to remove the most loosely held electron from a single neutral atom (or ion) in the gas phase, turning it into a positive ion. It is usually quoted as the first ionisation energy, for plucking off the first electron; removing further electrons takes successively more energy, giving second, third, and higher ionisation energies.
It is a direct, measurable handle on how tightly electrons are bound, so it tracks the periodic trends closely: it rises across a period (stronger effective nuclear charge) and falls down a group (outer electrons are farther out and better shielded). The caveat is the trend has telltale bumps — for instance, it dips slightly going from group 2 to group 13 and from group 15 to group 16, fingerprints of subshell structure.
Sodium gives up its first electron very easily (about 496 kJ/mol), because that lone 3s electron is loosely held. But its second ionisation energy leaps to nearly ten times higher, because that electron must be torn from the snug, noble-gas-like inner shell — a jump that betrays sodium's hidden shell structure.
The energy to pull an electron off — higher means a tighter grip.
A huge jump between successive ionisation energies marks the point where you start removing electrons from a deeper, full shell — a clue to how many valence electrons an element has.