Helium balloons consistently deflate faster than balloons filled with air, which is odd, and often frustrating. You would think there is more importance in making a more secure seal for the helium balloon than the regular balloon to prevent any helium escaping, because it is lighter than air. Also, helium balloons are a bit more special (and expensive) than regular air balloons – so you would like to think there was a bit more care in their creation, and that they would last longer, but they don’t. Heluim balloons always go down, but it’s not a case of being ripped of with a cheap balloon.
A helium balloon, will contain pretty much 100% helium (surprisingly) and helium is a very small atom. In fact, helium is the 2nd smallest atom on the periodic table. In addition to this helium is found in a monoatomic form in gas, which means it isn’t bonded to anything, which means it can move much more freely.
Air balloons on the other hand, are made up of 78% nitrogen gas (which is diatomic (N2)), and 21% oxygen (also diatomic) alongside small amounts of other molecules including carbon dioxide, argon and water vapour – all of which are also quite big molecules. These molecules are all made up of large atoms, and can diatomic, or even triatomic in the case of carbon dioxide and water, which makes them very large indeed, much much larger than helium.
As nitrogen and oxygen are by far the most abundant molecules in air, I’ll compare the size of these to helium. From looking at the above periodic table you can see that oxygen has an atomic number of 8, whereas helium is 2. This gives oxygen an atomic mass of 16, and helium 4. As oxygen is diatomic (2 molecules) in its gas form, the mass of the oxygen in the air balloon is 32, whereas the mass of helium in the helium balloon is still 4.
So, in short, the oxygen in the air balloon is 8 times bigger than helium, and this means that helium can diffuse through the balloon wall much easier than oxygen. Nitrogen is right next to oxygen on the periodic table, and so is a similar size to oxygen, and will find diffusing through the balloon wall just as difficult.
The material of the balloon is key to allowing this diffusion to happen. Most balloons are made up of an elastic polymer, which a mess of long strands all tangled together. This allows the balloon to have elastic properties, but also allows the helium to escape. These polymers will pack together very similar to a pile of spaghetti. Although it’s quite dense, there is are small holes, and small molecules like helium can easily slip through them. If you imagine the balloon wall to be a pile of spaghetti, then sprinkling peas on it would be like helium gas coming into contact with it – some would easily fall through the spaghetti. Nitrogen and oxygen will be more like meatballs, which would make contact with the spaghetti at multiple points, and struggle to pass through any gaps.
No matter how tight you tie the end of a balloon, helium balloons will always deflate faster than regular air balloons. This is because the helium atoms are many times smaller than the oxygen and nitrogen molecules which inflate air balloons, and so are able to diffuse through the holes in the balloons skin much easier. This mean more gas can escape the helium balloon than the air balloon in any given time.
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The helium molecules are so small, they can pass through the balloon wall – causing it to deflate.