We know that molecules which are made up of one or more atoms constitute matter.
In solids these atoms and molecules are rigidly fixed and space between then is very less of the order of few angstroms and hence they can not move.
.In liquids the atoms are not as rigidly fixed as in solids, and can move around. This enables a liquid to flow
In gases atoms are free to travel without colliding for large distances such that if gases were not enclosed in an enclosure they would disappear.The average distance a molecule can travel without colliding is called the mean free path. The mean free path, in gases, is of the order of thousands of angstroms
In solids and liquids the closeness makes the inter-atomic force important. The force has a long range attraction and a short range repulsion. The atoms attract
when they are at a few angstroms but repel when they come closer
In gases, inter-atomic force can be neglected
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