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    Oxygen is a chemical element – a substance that contains only one type of atom. Its official chemical symbol is O, and its atomic number is 8, which means that an oxygen atom has eight protons in its nucleus. Oxygen is a gas at room temperature and has no colour, smell or taste. Oxygen is found naturally as a molecule. Two oxygen atoms strongly bind together with a covalent double bond to form dioxygen or O2.


    Ozone is another form of pure oxygen. It forms in the Earth’s stratosphere. Ultraviolet light (UV) splits O2 molecules into single oxygen atoms. The single oxygen atoms latch onto Omolecules to form O3 (the chemical formula for ozone). The stratosphere has higher concentrations of O3 because there is more UV present. The ozone layer filters out UV, which reduces the opportunity to split Omolecules in the lower atmosphere (troposphere), where we live. Ozone can still form in the troposphere when Ois put under high heat and pressure. Car engines have the right conditions to produce ozone, which is a toxin, so modern cars use catalytic converters to convert O3 back to O2.

    Liquid oxygen and solid oxygen

    When oxygen is cooled to -183℃, it becomes a liquid. Liquid oxygen is used as a propellant for rockets – including the Rocket Lab’s Electron rocket. Oxygen becomes solid at temperatures below -218.79℃. In both its liquid and solid states, the substances are clear with a light sky-blue colour.


    Oxygen is everywhere!

    • It is the third most abundant element in the universe after hydrogen and helium.
    • It is the most abundant element by mass in the Earth’s crust. Oxygen forms compounds with silicon and other atoms to form minerals like quartz and sand.
    • It is the second most abundant gas in the atmosphere. About 21% of air is oxygen. However, oxygen is very reactive. It easily combines with other elements. It is replenished by plants and other photosynthetic organisms.
    • Oxygen also makes up most of Earth’s oceans by mass. A water molecule (H2O) is made up of two hydrogen atoms and one oxygen atom. Although there are more hydrogen atoms in water, hydrogen has less mass. By mass, water consists of nearly 90% oxygen.
    • It is the most abundant element in the human body (by mass) because our bodies are 70% water.

    Oxygen means life

    Oxygen is vital for life. Most living things use oxygen for cellular respiration – the process by which cells obtain energy. Humans and other land animals breathe oxygen (as part of air) into lungs. Here it is absorbed into the blood and carried to the body’s cells. Fish use gills to obtain dissolved oxygen from the water. Insects get their oxygen through small external openings called spiracles, which lead into a network of tubes called trachea.

    Plants also use oxygen for cellular respiration. In a separate process – photosynthesis – plants take in carbon dioxide and give out oxygen. In low light, the photosynthesis rate is less than the respiration rate. In bright light, the photosynthesis rate is greater than the respiration rate and more oxygen is produced than used. The excess oxygen is released into the atmosphere.

    Nature of science

    Words we use every day can have different meanings in science. ‘Respiration’ is commonly used to mean breathing – the inhalation and exhalation of air. In biology, cellular respiration refers to the chemical reactions in cells that release energy from food. It is important to discuss words like respiration to avoid misconceptions. 


    Oxygen, by itself, does not burn, but it is necessary for combustion. Oxygen is one of the three components of the fire triangle. Fuel and heat are the other two components. Heat generated by the fuel sustains the fire. As long as there is enough fuel and oxygen, a fire will continue to burn. Water-based fire extinguishers work by removing heat from the fire. Carbon dioxide and dry chemical extinguishers work by smothering the fire – cutting off the supply of oxygen.

    Activity ideas

    Explore oxygen with these activities:

    Explore the periodic table of elements with these activities:

    These articles provide background information about elements and how they are grouped:

    Useful links

    Visit to find out about the discovery of oxygen and oxygen facts.

    Visit the International Year of the Periodic Table (IYPT2019) website and then play Get in Your Element to earn a certificate and gain the chance to move on to the Nobelium Contest.


      Published 24 January 2019, Updated 18 February 2019 Referencing Hub articles