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How do particles move differently in liquids than in solids?

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argon phase diagram — liquid Figure 1: Phase diagram of argon.

On an atomic level, the fundamental difference between liquids and solids lies in the mobility of their constituent particles.

In a solid, atoms or molecules are not mobile. They are constrained to stay close to fixed points in space, where they oscillate rapidly due to thermal energy. This fixed, durable arrangement gives a solid its definite shape and resistance to shear stress.12

In contrast, particles in a liquid are mobile and continually wander throughout the material. While liquids maintain a high density similar to solids, they lack an enduring spatial arrangement. Because liquid particles are free to move and diffuse, they do not hold a definite shape and instead conform to the shape of their container. This mobility allows a liquid to flow and exhibit physical compliance, meaning it can yield to external forces, such as an object being inserted into it.123