Helium

• At ambient temperature and atmospheric pressure, helium is a gas. Helium has a density (the ratio of its mass to its volume) that is less than that of air; consequently, balloons filled with helium rise.
  
  

Gases

• Gases basically consist of molecules that are moving very fast and that have no appreciable attraction to other nearby molecules. The movement of the molecules is very chaotic, meaning that the molecules frequently collide with each other and with the sides of their container. One of the unique characteristics of gases is that they expand to occupy their entire container. Thus, helium molecules inside a balloon would evenly occupy the entire interior space.
  
  

Pressure

• The "pressure" inside a helium balloon is really the force being exerted when the helium molecules collide with the balloon's interior walls. Therefore, as those collisions become more energetic, the pressure inside the balloon rises, and as they become less energetic, the pressure falls.
  
  

Temperature Effects

• The final piece of this puzzle pertains to temperature. As temperature increases, molecules move faster, and as their velocity increases, so does their kinetic energy. Furthermore, as their temperature decreases, so does their kinetic energy. Thus, the force that gas molecules exert against the inside walls of their container--the pressure--is directly proportional to the temperature of the molecules. This is known as Gay-Lussac's law.
  
  With respect to a helium balloon, as the temperature of the gas molecules decreases, the molecules move more slowly and exert less pressure against the inside walls of the balloon. Thus, the balloon appears to deflate.
  
  

Effusion

• A second phenomenon can also account for a helium balloon deflating. Through the process of effusion, the helium molecules can pass through the balloon's pores. This is a consequence of the helium molecule's diminutive size. For effusion to occur, the molecules must be smaller than the pores they are attempting to pass through. Gases consisting of larger molecules, such as oxygen or nitrogen, would either effuse more slowly or not at all.