Why is potassium (K) is the most reactive metal of all metals? How can we judge the reactivity of metals?

Potassium is in the most reactive group of elements, the alkali metals, but it's not the most reactive metal within the group.

The alkali metals, Group 1A, are the most reactive metals because they have one valence or outer electron. They lose this electron very easily, forming ions with a charge of +1. Within the alkali metals family, reactivity increases with increasing atomic number. This makes Francium the most reactive, followed by cesium, rubidium, potassium,...

Potassium is in the most reactive group of elements, the alkali metals, but it's not the most reactive metal within the group.

The alkali metals, Group 1A, are the most reactive metals because they have one valence or outer electron. They lose this electron very easily, forming ions with a charge of +1. Within the alkali metals family, reactivity increases with increasing atomic number. This makes Francium the most reactive, followed by cesium, rubidium, potassium, sodium and lithium. Francium is almost non-existent in nature so cesium is the most reactive metal of those observed.

The reason for the trend of increasing reactivity with increasing atomic number within the alkali metals family has to do with the increasing number of electrons. Each element going down has an additional full electron energy level. Inner electrons repel the outer valence electron, making is less attracted to the nucleus and easier to remove. Francium is the largest atom within the group and has the least attraction between its valence electron and the nucleus so it loses an electron more easily than the other alkali metals. This same strong reactivity because of one valence electron is true of potassium, as well.