Strontium has two valence electrons. It lies in the family of elements called the alkaline earth metals. Strontium never exists as a pure element in nature and is usually found in the minerals celestite and strontianite.
When exposed to air, elemental strontium reacts with oxygen and turns from a silvery hue to a yellowish color as strontium and oxygen form strontium oxide. Because strontium has two electrons in its outermost energy level, it acts as an electron donor. To attain a more stable configuration, atoms want to fill their outermost energy level, which is usually eight electrons. Strontium prefers to lose two electrons instead of having to gain six electrons; therefore, strontium is an electropositive element and is considered an electron donor.