The reason that particles of gas are able to move freely around is because most gases in themselves have week bonds to each other and even so with those bonds the gas has large amounts of energy to break those such bonds. Ex: When you boil water, what do you use to boil it? Heat, and heat is energy you add heat to the water energizing the water allowing the water to jump up and away form it turning it into gas, but since it is now gas and charged it has free roam.
