The forces of attraction between gas molecules is too weak for them to mantain any kind of definitive shape.( solids only have a shape because there are strong intermolecular forces between solid molecules). The reason for gas molecules not being able to mantain a fixed volume is also the same, the inter atomic forces are too weak to prevent spreading of gas molecules.
Your looking for ( gas )
A gas is a state of matter that does not have a definite shape or volume. Its particles move freely and independently, taking up the shape and volume of their container.
The liquid phase
Nope!! No definite shape or size!
"solids"
Only solid matter has a definite shape
A gas does not have a definite shape or volume.
Gas tends to take the shape of the volume it's in. If the volume is reduced, then the pressure of the gas will increase.
liquid has no definite shape and a solid has no definite shape or volume
The matter that can change shape and volume is gas.
Gas will take the shape of whatever container it is in. One of the characteristics of a gas is that it has no definite shape or volume.
A gas has no fixed volume or shape.
A gas is a substance with no definite shape or volume.