| MadSci Network: Physics |
The nucleus of an atom contains protons, which are positively charged, and neutrons which have no charge. The electrons are negatively charged. Electrostatic forces drive the electrons and nucleus together, so in a classical model, they ought to stick to each other like glue to make our overall neutral atom. This is not the case, however.
These particles I am describing are so very small that we can't treat them like, say raisins and baseballs. The energy states of electrons are not continuous, but discrete levels called quanta. Ground level is the term to describe the lowest energy state of an electron. (By the way, baseballs have quanta, too, but the energy levels are so close together we are not able to notice, and are therefore effectively continuous).
But, even in this low-energy state, the electron doesn't just sit still. It always has a minimum energy. It possesses properties of both a particle and a wave. In addition, the Heisenberg Uncertainty Principle tells us that we can't know the exact location and velocity simultaneously.
We do our best, however, with wave functions derived from Scrodinger's equation. These functions enable us to determine where an electron is, say 90% of the time. This is what the orbitals (or "clouds") are, just probability distributions (think of the familiar bell curves teachers use to grade, then extend to 3-D). Different energy levels yield differently-shaped clouds. If you define the atom radius as the farthest point the electron goes in a 90% distribution, then there's a copious amount of free space in an atom.
To answer your question, then, the atomic radii which you are probably familiar with correspond to the average radius in the ground state for a probability of > 90%. Using radii is more for our convenience than anything else, since most of the space is not being used. The way we think of (and teach) atomic structure is our best interpretation so far, and it presents the atom in language and images that we can comprehend.
Try the links in the MadSci Library for more information on Physics.