Oxygen

1. Symbol O. A nonmetallic element constituting 21 percent of the atmosphere by volume that occurs as a diatomic gas, O2, and in many compounds such as water and iron ore. It combines with most elements, is essential for plant and animal respiration, and is required for nearly all combustion. Atomic number 8; atomic weight 15.9994; melting point -218.4 degrees C; boiling point -183.0 degrees C; gas density at 0 degrees C 1.429 grams per liter; valence 2. Some old people must use an oxygen mask to help their breathing.