Final answer:
Adding more gas particles to an air tank results in increased pressure due to more frequent and forceful collisions against the container walls. For a rigid container, the pressure would roughly double if the amount of gas particles is doubled, provided temperature remains constant.
Step-by-step explanation:
When you add more gas particles to an air tank, the pressure of the gas in the tank changes due to an increase in the number of collisions against the tank walls. Each gas particle exerts a force upon collision, and more particles result in more collisions and hence a greater overall force on the walls. Since the area of the container walls remains constant, the increased force results in increased pressure. This is under the condition that temperature is constant; if the gas also heats up, the increased speed of the particles will amplify this effect.
If the container cannot expand, like a rigid tank, then adding more gas simply increases the frequency of molecular strikes against the container's walls, and thus the pressure doubles if the amount of gas doubles. On the other hand, a flexible container like a balloon may not show a doubling in pressure when the amount of gas doubles, because it can expand.
There's also a consideration of temperature. More particles at a higher temperature move faster and will further increase the pressure inside the container. Similarly, atmospheric pressure does apply to any container, but it doesn't alter the pressure of the contained gas unless the container is flexible, like a toy balloon, in which case the outside pressure affects the total volume and therefore the gas pressure inside.