For my forth link I chose one that breaks down how atomic mass can effect how far the balloon could get. Atomic mass is the total mass of protons and neutrons in an individual atom or isotope. Atomic weight is the average mass of all of the naturally-occurring isotopes of an element. This video really goes into depth about how atomic weight and atomic mass are not the same.
For my third link I chose another one on how far a helium balloon could get in the atmosphere to compare the two. In this link the explanation goes into depth on how a balloon really can get so high into the atmosphere. It touches on drag force will limit the balloon's ascent velocity and the temperature and pressure.
For my second link I have chosen how far a helium balloon get in the atmosphere. This link gives a short explanation on how far a helium balloon can rise. From this article it states it can only get 32 kilometers before the density of the air matches the density of the helium.
For my first link I have chosen the layers of the atmosphere. This link provides information about the layers of the atmosphere and the extreme temperatures they hold. This would effect the helium balloon, because the temperature could freeze or melt the material the helium is in or cause it to expand or contract.