How small is an atom?

I send out a “Thought For The Week” and this week’s thought is as follows:

“The range of sizes, distances or speeds with which our imaginations are comfortable is a tiny band, set in the midst of a gigantic range of the possible, from the scale of quantum strangeness at the smaller end to the scale of Einsteinian cosmology at the larger”.
Richard Hawkins in “The God Delusion “ (2006)

Now, as regards “quantum strangeness”, it happens that right now I’m reading a book by John Gribbin called “In Search Of Schrödinger’s Cat” and subtitled “Quantum Physics And Reality” (if you don’t know about Schrödinger’s cat, check out the paradox here).
Now one of the things Gribbin manages to do is convey some idea of the smallness of the atom. He explains that a typical atom is about 10 to the power of minus 10 metres across. That is, 0.0000000001 metres.
Think that’s small? Try this then. At the heart of an atom is the nucleus (which contains the protons and neutrons). The size of a nucleus is about 10 to the power of minus 15 metres across. That is, 0.000000000000001 metres. That means that a nucleus is 10 to the power of 5 times smaller than an atom. That is, 100,000 times smaller.
Since volume is proportional to the cube of radius, it is more meaningful to say that the nucleus is 10 to the power of 15 times smaller than the atom. That is, a thousand million million times smaller.
Now that’s small.


  • Philip

    Strangely, I can only start to comprehend how small something is if it is scaled up.
    For me, Bill Bryson’s quote about atoms was helpful:
    “Neutrons and protons occupy the atom’s nucleus. The nucleus of an atom is tiny — only one-millionth of a billionth of the full volume of the atom — but fantastically dense, since it contains virtually all the atom’s mass. As Cropper has put it, if an atom were expanded to the size of a cathedral, the nucleus would be only about the size of a fly — but a fly many times heavier than the cathedral”.

  • Roger Darlington

    That’s a good illustration, Philip.
    In fact, Gribbin has very similar one. He states that, if the neutron was a pin head in the middle of a cathedral, then the dust motes in the highest part of the ceiling would be the electrons in the outermost sections of the atom.

  • Ellis Dallal

    Hi Roger,
    Good to see you yesterday. If you imagine the atoms and nuclei of our universe as the planets and galaxies of another infinitessimally small universe, and if you imagine our planets and galaxies as the atoms and nuclei of another garguantually large universe, you could go on forever with these ‘nested’ universes. My totally unscientific but to my mind ‘neat’ theory!

  • Roger Darlington

    Good to see you and Jo, Ellis.
    Your ‘nested’ universes image recalls to mind a scene at the end of the film “Men In Black” when our universe turns out to be a marble in a game by mega-giant aliens. Who knows?