* Ionization Energy: This is the energy required to remove one electron from a neutral atom in its gaseous state.

* Reactivity: Metals are reactive because they tend to lose electrons easily to form positive ions (cations). This makes them good reducing agents.

* Relationship: The easier it is to remove an electron (lower ionization energy), the more readily the metal will form a cation and participate in chemical reactions.

Example:

* Alkali metals (like lithium, sodium, potassium) are extremely reactive. They have very low first ionization energies because their outermost electron is relatively loosely held.

Therefore, a very reactive metal would likely have a first ionization energy in the range of 5-10 electron volts (eV).