Air is approximately 21 percent oxygen gas (O2) and 78 percent nitrogen gas (N2). Can you tell from the information in the periodic table why helium balloons float in air?

Helium exhibit less density as compared to oxygen and nitrogen thus the balloons floats in the mass of air.

Explanation:

Oxygen has an atomic mass of 15.994 and nitrogen has an atomic mass of 14. 006. The helium which is a noble gas has an atomic mass of 4. 002. This is indicative of the fact that the helium is less denser than the air containing oxygen and nitrogen. Thus as a result of this the helium floats on the air being lighter then the other components of air and hence, the helium balloons float in the air.