Question

Why does a gas have no definite shape and no definite volume?

Answer 1

Hint: There are three main states of matter - solids, liquids, and gases. All matter consists of molecules that have spaces between them called intermolecular space. The extent of this intermolecular space is the defining factor for the properties of the state of matter.

Complete answer:
For having a definite shape the molecules should be fixed or have a definite arrangement, but that’s not the case for the gases. In gases, the intermolecular space is relatively larger, the rigidity is not observed and the kinetic energy is also low for them.
The kinetic energy is the function of the distance between the molecules, the energy is inversely proportional to the distance between them, as the intermolecular distance is larger, so kinetic energy is lower, thus the interaction between the molecules is insufficient to maintain the volume and shape.
In gas, the molecules are loose. Compared to solids and liquids, the attraction between gas molecules is the smallest. The kinetic energy between molecules is high enough to move away from any limit and move randomly. Therefore, the gas has no definite shape or volume.

Note:
The solids have a fixed shape and volume while the liquids acquire the shape of the vessel that contains the liquid. The liquids have a definite volume. The definite shape and volume are because of stronger intermolecular interactions and hence higher kinetic energy.