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Q & A: Why are atoms so small?

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Most recent answer: 10/22/2007
How come the atom is so small?
- Anonymous
Another way of asking this question is: Why are you so big? If we were smaller ourselves, we wouldn’t think of atoms as being so small. But atoms come in fixed sizes -- we cannot shrink ourselves down and also shrink the atoms down. Smaller organisms are just made up of fewer atoms.

We are complicated beings -- our bodies do lots of amazing things every day. Just to ask a question about atoms requires a brain that can process language and think about what things are made of. This takes lots of reactions among neurons, and lots of supporting roles by other parts of the body to keep it all working. In short, lots of atoms are needed to make a human body as complex and as efficient as it is. It is natural then for us to see atoms as small. A virus typically is made up of far fewer atoms but it cannot ask the question you just did.

If you were to ask why atoms are the size they are given what they're made of, we could answer in terms of some basic constants that influence how the little quantum clouds get held together. But even if those constants were different, you'd still have to put together a huge number of the atoms to make anybody who could ask about their size.

Tom, w mods by mbw&jjt

(published on 10/22/2007)

Follow-Up #1: Sizes of atoms

What determines the size of an atom?
i.e. What makes all protons,neutrons,electrons, etc.(in every element) the same size?
i.e. Why can’t you have 2 atoms of the same element but one twice as big as the other. Or even millions of times bigger?

eg. Imagine a single atom in space. What size is it seeing as there is nothing to measure it against?
Now imagine the first is gone and there is another bigger atom. Is it the same size as the first one? (relative to nothing)
can these 2 atoms coexist?

Does this question make sense or am i being erm...a little bit stupid =)
- Jeremy Fisher (age 19)
Central london, uk
Your questions are certainly not stupid.  They occupied the minds of leading physicists in the early 20th century.  Rutherford determined that the atom consisted of a heavy positively charged core, the nucleus, surrounded by a ’cloud’ of negatively charged electrons.  A big puzzle was why didn’t the electrons just spiral down and be swallowed by the nucleus.  The answer has to to do with quantum mechanics. The resulting ’size’ of the atom is then determined by the ’Bohr radius’ which is an appropriate collection of fundamental constants: the mass of the electron, the charge of the electron, and Plank’s constant. You can check out the Wikipedia article for more information on the Bohr radius.

Now as to measuring the size of an atom;  it is straightforward to measure the spacing of atoms in a crystal by scattering x-rays from it and looking at the interference pattern.  Measuring the size of a single atom in free space is a bit more tricky but in principle one can imagine doing it in a statistical sense, again by scattering photons from the atom.

Can single atoms of one species have different sizes?  Yes, but there is a smallest size, the so-called ground state or lowest energy state.   The larger sized versions are in what are called ’excited’ states.  If left alone they will eventually decay into the ground state by emitting photons. 


(published on 10/22/2007)

Follow-up on this answer.