Quote:
Originally Posted by Houston
going down the periodic table, the size increases (thius should be obvious. you have more protons and neutrons, hence a bigger neuclues, and more electrons around it)
Going from left to right, the size decreases. Why? because as you go across from left to right, you have more protons in the nucleus. This in turn pulls the electrons tighter to the nucleus (since protons are positively charged, and nuclei are negatively charged)
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The first explanation (going down the periodic table) can't really be obvious, because it's contradicted by the second explanation, which says more protons = smaller atoms.
What I'd say is that as you go from one element to the next down a group you are adding a new shell of electrons. It's this extra shell that makes the atoms bigger going down a group.
As you go along any period from left to right you are dealing with the same electron shell which is pulled smaller by the extra charge from more protons in the nucleus.
Here's a neat pic from
Dr. Tom Zamis, University of Wisconsin.