Spaces and Forces: The differences observed in solids, liquids, and gases can primarily be attributed to the arrangement, spacing, and intermolecular forces acting between the molecules.

Solids: Tightly packed molecules with strong intermolecular forces, giving a definite shape and volume.
Liquids: Close molecules that can move past each other, resulting in a definite volume but no definite shape.
Gases: Widely spaced molecules with weak intermolecular forces, filling the entire container.