The molecular weight of potassium is 39.0983 g/mol. It is a very light and reactive alkali metal. Potassium, which is very soft, is the second least dense metal next to its fellow alkali metal, lithium.
Potassium's low molecular weight means that although it is the seventh most common metal on Earth, it only makes up 1.5 percent of the Earth's crust's weight. Because it is so reactive, it is never found alone in nature; it is always bonded with something else. Pure potassium can ignite when exposed to large quantities of water. It must be stored under argon or mineral oil to prevent oxidation upon exposure to open air.