What makes the earths atmosphere a heterogeneous mixture?

A heterogeneous mixture is the mixture which is made up of two or more substances in a undefined ration or proportion. The Earth’s atmosphere is a heterogeneous mixture because density of various components of gases is different in Earth’s atmosphere. The heavier gases such as CO₂ and O₂ of the atmosphere are present near the earth’s surface of the atmosphere whereas lighter gases like O₃ i.e. Ozone gas occupy the higher level of the atmosphere and therefore, the components of atmosphere are not distributed in equal amount.

So, the components of atmosphere are not equally distributed.