Hi Lin- I've taken your question out of the thread http://van.physics.illinois.edu/qa/listing.php?id=1295
because it was getting way too long.
The ingredients of an atom are a heavy, compact electrically charged nucleus and enough oppositely charged electrons (much lighter) to make the total charge zero. Since opposite electrical charges attract, just as masses attract by gravity, you'd guess that maybe an atom was kind of like a little solar system, with light electrons orbiting around the heavy nucleus like planets around the sun. Early in the 20th century, it was clear that that guess wasn't right. Charged particles going in circles are just like the currents in a transmitting radio antenna. They radiate energy. You can calculate how much, and it turns out it's enough to make an atom collapse in about a billionth of a second.
By 1912, Planck, Einstein, Debye and others had noticed that there seemed to be a strange lumpiness in the way energy was transferred between light and sound and other things. In 1913, Bohr suggested modifying the planetary model by assuming a similar lumpiness in what sort of orbits were allowed. Instead of being able to rotate at any speed, only certain speeds were allowed.
This Bohr picture made no sense for ingredients like little particles but it reproduced some of the features of small atoms very well. It was a step on the path to quantum mechanics, which gives an accurate description of atoms. In quantum mechanics, however, the ingredients are not anything like little particles with positions and velocities. The ingredients are all quantum waves, whose behavior is not like classical waves.
Maybe that's past where you asked me to stop.
(published on 10/30/2011)