Potassium (element), kalium
鉀 centers on the chemical element potassium, also known by its older name kalium. The meaning is a single specific element, not a general concept.
鉀 combines the metal radical 金 with the phonetic component 甲 (コウ). It was created as a character for the element potassium, using 甲 to suggest the sound and 金 to indicate a metal.
The metal radical 金 plus 甲 (which looks like a helmet or armor) suggests a metallic element. Think of potassium as a reactive metal that needs protective armor.
For コウ, imagine a coat of armor made of potassium: coat -> コウ, and the metal 金 needs a protective coat.