Here's why:

* Mixture: A mixture is a combination of two or more substances where each substance retains its individual properties. Air is a mixture because the nitrogen, oxygen, and carbon dioxide molecules are not chemically bonded together.

* Elements: Elements are pure substances that cannot be broken down into simpler substances by chemical means. Nitrogen and oxygen are elements.

* Compound: A compound is formed when two or more elements combine chemically in a fixed ratio. Carbon dioxide is a compound because it's formed by the chemical combination of carbon and oxygen atoms.

While oxygen and carbon dioxide are significant components of air, nitrogen is the most abundant gas in the atmosphere, making up roughly 78% of the air we breathe.