Fluorine is the only element that is more electronegative than oxygen. Wood and rubber ignite spontaneously in gas, and even asbestos becomes incandescent. Copper and steel are attacked by , but are quickly protected by adhering layers of and , reminiscent of the protective coating on aluminum. The halogens all are too reactive to exist as pure elements in nature, and usually are found as their ON = -1 salts. They form compounds with every known element except the noble gases.