A childhood belief about chemistry

Early in my becoming acquainted with science, I thought the density of all elements would be proportional to their atomic weights, regardless of whether they were solids, liquids, or gases. I "figured out" some amazing things based on that. For one thing I wanted to do a demonstration that I never got to do because of the difficulty of obtaining the heavy gas xenon. I thought that if only I could get some xenon and fill a beaker with it, I could do an amazing demonstration. For one thing I thought the xenon would be so heavy that it would stay in the beaker without dispersing into the atmosphere for quite some time. Then I thought I'd place a piece of iron on top of the xenon in the beaker and see the iron float on the xenon. Since xenon has a considerably higher atomic weight than iron, I assumed that even gasseous xenon would be denser than solid iron, resulting in that floating of the iron that I expected. When I later learned about the behaviour of gasses, I realized that gasseous xenon, for all its large atomic weight is much less dense than most any solid, including iron, so likely no solid would float on it. And even a heavy gas would disperse into the atmosphere quickly, so the xenon wouldn't likely stay in the beaker long enough for me to even try the demonstration.
Another amazing "fact" I deduced on the same basis came after I first heard of osteoporosis, and how it is a disease causing loss of bone density. I knew that bones have a lot of calcium, an element of considerably larger atomic weight than nitrogen and oxygen that make up most of the atmosphere. But human flesh, I learned, is composed primarily of hydrogen, carbon, nitrogen, and oxygen. Since hydrogen and carbon have lower atomic weights than the latter two that also make up air (99% or air, anyway), I deduced that human flesh must be lighter than air, so that our heavier bones were what holds us down. So I thought that the main risk of osteoporosis sufferers was that their bones could become so depleted of calcium that their bodies' overall density would be less than that of air, and they could go floating away to the top of the atmosphere!