Which elements are all highly reactive and are never found in Elemental forms in nature? Because of this, they are usually stored in mineral oil or kerosene (paraffin oil).
These elements are called the alkali metals. (You could also say the alkaline earth metals).
The elements in this group are not found freely in nature (only in the minutest traces), primarily due to their high reactivity. They readily lose their loosely-bound outer electron to form ions and compounds. When exposed to air, these elements will immediately react with oxygen (the metal is oxidized) to form metal oxides, which makes the substance appear tarnished. When placed in water, the elements react vigorously to produce dissolved metal hydroxides and hydrogen gas (a cool experiment to watch, by the way!)
To prevent these reactive metals from reacting with the oxygen, the pure metals are often stored in oils. In nature, these metals occur only in salts (metal-nonmetal compounds), such as
Alkaline earth metals are also quite reactive and rarely found in nature, but the alkali metals are much more reactive. They both are stored in oils, actually, to prevent their oxidation.