Some highly reactive elements are fluorine, sodium, potassium, lithium and oxygen. Reactive elements release great quantities of energy when they combine with other elements. Some tarnish in air or explode in water.
Fluorine is an extremely reactive pale yellow-green gas. It reacts violently with nearly everything, including metals, steel wool and even glass. However, when fluorine is combined with other elements, the result is extremely stable. One of these materials is Teflon, which is almost impervious to chemical attack.
Sodium is so reactive that it explodes when it comes into contact with water. This can produce flammable hydrogen gas or sodium hydroxide. Though it's common in the Earth's crust, sodium is never found in its pure state but needs to be isolated from compounds.
Potassium, an element vital to good health, is even more reactive than sodium. It also explodes if it's exposed to water, creating purplish flames. Potassium quickly turns black if it's exposed to air. It's crucial for the proper functioning of nerve cells in the body.
Lithium doesn't explode in water, but is so light and soft that it floats. Contact with air causes it to tarnish quickly. Oxygen has the ability to react with a great variety of organic compounds and is necessary for life.