What metal is the most reactive metals in it's period and cannot be found in uncombined in nature. Each atom of the element contains 19 protons?

The metal you are referring to is potassium (K). It is the most reactive metal in its period (period 4) and cannot be found uncombined in nature due to its high reactivity. With 19 protons, potassium readily reacts with other elements, particularly nonmetals, to form compounds.