How does ionization energy relate to reactivity?
What is reactivity?
Reactivity refers to how likely or vigorously an atom is to react with other substances (atoms, molecules, etc). This is usually determined by how easily electrons can be removed (we call it ionization energy!) and how badly they want to take other atom's electrons (we call it electronegativity).
Elements with high electronegativity will be very reactive, as will elements with low ionization energy. Alkali metals, for example, are very reactive but there is difference between sodium and ceasium. Ceasium has lower ionization energy so it is VERY reactive. Sodium has greater ionization energy than ceasium so it is not so reactive as caesium but it's still very reactive.