The volume of a gas is defined as the size of the container in which the gas is found. This is very different from the volume of liquids and solids. If a small amount of water is placed into a 2-liter soda bottle, the water will collect at the bottom of the bottle and have the same volume it did before it was poured into the bottle. However, if a small amount of helium is added to the bottle, the volume of the helium will be 2 liters.
Essentially the volume of a gas is the volume of space in which you might find a particle of the gas at any particular moment. As an extreme example, imagine putting only one atom of helium into the bottle. The atom of helium would bounce around the bottle, so that there is not place in the bottle, where the atom could never be. Therefore the volume of the helium is the entire container.
NOTE: the volume of the helium ATOM is still insanely small – it is, after all, a tiny particle. The volume of the helium GAS is 2 liters.
Even more disturbing, if a two liter bottle contains only nitrogen, the volume of the nitrogen is 2 liters. If oxygen is then added to the bottle (without removing the nitrogen) the volume of the nitrogen is STILL 2 liters. The volume of the oxygen is also 2 liters and the volume of the mixture of the two gases together is 2 liters.The volume of a gas could be measured in any unit ranging from gallons to teaspoons but generally we record volume in liters.