physic問

There is intermolecular force between molecules. This intermolecular force is generally attractive in nature and its magnitude decreases with increase of separation between molecules.
Molecules in solids are closely packed. Hence, the separation between molecules is small. The intermolecular force is strong enough to hold the molecules together, such that molecules can only vibrate about a mean position, but with their separation remains more or less the same. This makes solids have a definite shape and geometry.
When the solid substance is heated to become liquid, molecules acquire enough energy to do work against the intermolecular force to increase the molecular separation. In so doing, the attractive force between molecules decreases, and the molecules can move more freely than in solids. The substance then has no definite shape because of the free movement of molecules. However, intermolecular force still have influence on the molecules so that the liquid appears as a fluid-like substance under the attraction of gravity.
In the gas state, molecules have (kinetic) energy high enough to free almost completley from the influence of intermolecular force. They can fly apart freely in space. The attractive force between molecules is negligibly small. The molecules can fill up everywhere in the space at which the gas occupies.

It is because solids particles more close and they have 5記得弱作用力or強作用力 let particles not easy to move out.

But gases and liquids,there particles are not closer the solids.