Ionization Energy
Definition
The energy needed to pull an electron completely off an atom. Atoms that hold electrons loosely have low ionization energy and give up electrons easily.
Symbol: · Unit: kJ/mol
Technical definition
Ionization energy is the minimum energy required to remove an electron from a gaseous atom or ion, forming a cation. First ionization energy generally increases across a period and decreases down a group, reflecting effective nuclear charge and atomic size.
Example
Sodium's first ionization energy (496 kJ/mol) is far lower than neon's (2081 kJ/mol): sodium happily becomes Na+, while neon refuses to ionize.
First, second, and third ionization energies are very different. Removing a second electron always costs much more because you are pulling a negative electron away from an already positive ion.