Here's why:

* Reactivity in metals increases as you go down a group (column) in the periodic table. Potassium is in Group 1, the alkali metals, and is below sodium.

* Reactivity also increases as you go from right to left across a period (row). Potassium is on the far left side of the periodic table.

This means potassium has the highest tendency to lose its outermost electron, making it the most reactive metal in the first 20 elements.