How big is an atom?

I started back at work on Monday thinking that it would be a nice, peaceful day, with no-one else around on campus. Surely, on a beautiful, sunny, 6th January, the entire of Hamilton except for myself would be on the beach at Raglan. Wow, was I mistaken. The campus was buzzing with activity and there was a constant stream of knocks on my door from people wanting things done. I've hardly had time to breathe, let alone write blog entries. Maybe things will quieten down when A-semester starts (!). 

On Sunday Benjamin (now 18 months old) acquired a balloon filled with helium. I've been wondering for a little while about letting him drop a helium balloon just to see the reaction when it fails to hit the floor, and I was given the chance when a friend gave us one. (Inidentally, the frivolous use of helium in party balloons is a subject that is worth debating in itself.) Our child was most impressed and rather excited to see that not everything obeys the law of gravity, or, at least, not in the way  he understood it. 

So, the balloon was up on the ceiling for a few hours. What surprised me, however, was how quickly it lost its helium. The following morning it was back on the floor, rather deflated. This probably shouldn't have surprised me – there's good reason that the helium escapes the balloon quickly – the helium atoms in the gas are very small, and very inert (non reactive). They can simply make their way out through very tiny holes in the rubber of the balloon. If the balloon were filled with air (about four fifths nitrogen molecules – two atoms of nitrogen joined together, and one fifth oxygen molecules) I'd expect it to stay inflated rather longer. The air molecules are so much larger than the helium atoms. 

Defining the size of an atom is a bit like defining the size of Auckland. Putting aside artifially drawn boundaries, where exactly does the urban development stop? Nonetheless, there are some practical definitions of atomic size. Helium has a diameter of about 60 picometres (1 picometre is ten to the power of minus 12 metres, that is, 0.000 000 000 001 of a metre). Contrast this to the size of a nitrogen molecule, which is (again, vaguely), about 100 picometres wide. The smaller helium is simply better at getting out through the holes in the rubber balloon, and so the thing doesn't stay inflated for long. Instead, some precious atoms are lost into the upper atmosphere.  








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