The defining characteristic of alkali metals is that they have only one electron in their outermost electron shell.

Here's why this is important:

* Reactivity: This single electron is relatively weakly held by the nucleus, making it easy to lose and form a +1 cation. This is why alkali metals are extremely reactive, readily reacting with other elements to form ionic compounds.

* Electropositivity: The tendency to lose this electron and form a positive ion makes alkali metals highly electropositive.

* Low ionization energy: Because the outermost electron is loosely bound, it requires relatively little energy to remove it, resulting in low ionization energies.

Let me know if you'd like more details about any of these characteristics!