If you released a helium balloon and if it never popped how high would it rise?
It would rise until the density of the balloon and contents equaled the density of the surrounding air that the balloon displaces. You may want to review "buoyancy" to see why this is so.
So would that be about the top of the earth's atmosphere?
No, the Earth's atmosphere gets thinner and thinner as altitude increases, with no real "top" to speak of. The balloon would rise until the mass of the balloon and the gas inside equal the mass of the volume of atmosphere that the balloon displaces.
IIRC Well over 90% of the mass of the atmosphere is within the first 10km but it continues for several hundred after that.
The balloon has 2 parts, the skin, and the helium within. The helium is less dense then anything except hydrogen. The skin will be more dense then the atmosphere. The higher the ratio of the balloons volume to its surface area the less its total density will be. The larger the balloon the higher that ratio will be.
I think this also depends on the type of balloon as well. A regular stretchy balloon you can blow up yourself should rise much higher, as the helium inside can cause the balloon to expand as it rises and the outside air pressure falls. A balloon you buy at the store that is silvery and says Happy Birthday (or similar) on it is typically not able to expand.
Given a massless skin, and an unlimited expansion rate ending up with over 1 km to 100 km between helium molecules, the balloon would rise higher than low orbiting spacecraft, between thermosphere and exosphere (mostly helium and hydrogen). If you released some helium (no balloon, just helium gas) into the atmosphere, then theoretically some of it could eventually end up in the exosphere. If you released some hydrogen, then theoretically some of it could reach the outer edge of the exosphere and some of those molecules could achieve escape velocity and leave the earth.
Weather balloons with volume expansion rates of 100:1 can achieve 40 km (25 miles) altitude.
If the skin of the balloon could expand infinitely the balloon would theoretically rise forever. For example. if the balloon rose to an altitude where the pressure was half as much the balloon would have twice the volume. That means it would displace twice as much air. But since the air is also at half the pressure the total weight of air displaced doesn't change. Since the lifting capacity of a balloon is equal to the difference in mass between the helium in the balloon and the mass of air that is being displaced the lifting capacity would remain constant as the balloon rose through the atmosphere. Therefore if, theoretically, the earth's atmosphere extended to infinity getting thinner and thinner the balloon would rise to infinity. Theoretically of course.
That is one huge "if"! It is so huge (as in: not true) that it does not belong at the end of your explanation; it belongs right at the beginning.
It's kind of like saying "theoretically the balloon could reach Andromeda", then going on for a couple of paragraphs, before finishing off with "if it had an interstellar drive attached to it."
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