The average distance a molecule can travel without colliding in a gas is called:

Molecular nature of matter:

• From many observations, in recent times we now know that molecules (made up of one or more atoms) constitute matter.
• Electron microscopes and scanning tunneling microscopes enable us to even see them.
• The size of an atom is about an angstrom (10-10 m). In solids, which are tightly packed, atoms are spaced about a few angstroms apart.
  • In liquids, the separation between atoms is also about the same. In liquids, the atoms are not as rigidly fixed as in solids and can move around. This enables a liquid to flow.
  • In gases, the interatomic distances are in tens of angstroms.
• 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.
• The atoms are much freer in gases and can travel long distances without colliding.
• If they are not enclosed, gases disperse away.
• In solids and liquids, the closeness makes the interatomic force important.
• The interatomic 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.
• The gas is full of activity and the equilibrium is a dynamic one. In dynamic equilibrium, molecules collide and change their speeds during the collision. Only the average properties are constant.
• Atoms consist of a nucleus and electrons. The nucleus itself is made up of protons and neutrons. The protons and neutrons are again made up of quarks.

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