A molecule of carbon dioxide contains two oxygen atoms.
The formula CO₂ states the molecule’s composition directly: one carbon atom is joined to two oxygen atoms. The subscript 2 applies only to oxygen, while the absence of a subscript after carbon means there is one carbon atom. Carbon dioxide therefore has three atoms in each molecule and is called a triatomic molecule.
The atoms form a straight, or linear, arrangement: oxygen–carbon–oxygen. The carbon atom is double-bonded to each oxygen atom, and the molecule’s symmetry means that carbon dioxide has no permanent electric dipole moment even though its bonds are polar.
Carbon dioxide is a greenhouse gas because it absorbs infrared radiation emitted by Earth’s surface. It occurs naturally through respiration, decomposition, volcanic activity, and the carbon cycle, while human activities such as burning fossil fuels add more. It is not the same as carbon monoxide, CO, which contains only one oxygen atom and is a different, poisonous compound.