What is the primary reason why gases have no definite shape or volume?

The primary reason gases have no definite shape or volume is due to the nature of their molecules, which move freely and are far apart from each other. In a gaseous state, the molecules possess significant kinetic energy, allowing them to move rapidly in all directions. This freedom of movement leads to a lack of fixed arrangement, resulting in gases expanding to fill the container they are in, irrespective of its shape or volume.

Unlike solids and liquids, where molecules are closely packed and have more defined positions due to stronger intermolecular forces, gases are characterized by minimal interaction between molecules. This not only accounts for their ability to change shape and volume based on the container but also explains why gases can compress or expand significantly compared to solids and liquids.