Oxygen is a chemical element made up of subatomic particles including protons, neutrons and electrons. The atomic number for oxygen is 8, which means it has eight protons and eight electrons. The number of neutrons varies, but the most common isotope of oxygen has eight neutrons.
In its natural state, an oxygen atom is commonly paired with another oxygen atom to form the diatomic molecule O2. When three oxygen atoms are bound together they form O3, which is the chemical formula for ozone. Oxygen is the third most abundant element in nature and is found in hundreds of thousands of organic substances, often in combination with hydrogen, carbon, nitrogen or sulfur.