Gases have no shape or true measurable volume unless it is compressed, and even then, measuring volume is different than just seeing ‘how much of the gas is in a container.’ The reason that gas has no true shape is because its molecules are far apart and unrestrained.
These molecules slip and slide past each other similarly to liquid but as mentioned, can be compressed, unlike liquid states of matter.
Gas has no specific shape or volume because the forces of magnetism between gas molecules are too weak for them to keep any form. Gas can spread throughout the container in which it is placed. If the container is large, the gas can extend widely and if the container is small, it can't.
The molecules move freely, and gases are compressible. With the help of a pump, they can be packed into a lower ambient pressure, and they will fill that space too. The particles of gas contain a lot of kinetic energy. The particles move very fast and strike one another, causing them to disperse until they are evenly dispensed throughout the volume of the container.
