Van de Graff Experience

On March 13th, 2009 we did a Van de Graff activity. The Van de Graff is a machine that stores electrons from a rubber band in a dome. One of the things we used on it was bubbles. We found that the bubbles moved away from the Van de Graff. I believe that they flew away because the electrons stored in the dome is a lot of static energy, and the bubbles are a neutral energy. Therefore the bubbles repelled from the large difference in energy.
Another activity we did was a human chain. One person touched the dome of the machine and several people held hands (one being the person holding hands with the person touching the Van de Graff machine). We found that no one in the human chain got shocked but if someone outside of the chain touched the chain they would be shocked. I believe that this was because the energy of the people in the chain balanced out with the electrons in the dome, and the people got shocked because there body hadn't adjusted to the electron transfer.
Another demonstration we did was one where we placed a napkin on the machine vs. several napkins on the machine. We found that the edges of the single napkin quickly lifted and the napkin flew off. However the stack of napkins did not move, only the edges flew up. I think this was because there were not enough electrons to push away the stack, but the single napkin was lighter and was able to be pushed.
Another example we used to learn was to stick strips of paper to the dome. We found that the pieces of paper stood up and repelled form one another. I believe this occurred because they both had the same amount of electrons so they could not be close to one another.
The last activity we did was to put a handful of Cheerios on top of the dome, and turn on the machine. We found that the Cheerios quickly hopped of the machine in random directions. I think this happened because as the dome filled with electrons the force of the electrons greatly exceeded the amount of electrons in the Cheerios, causing them to repel.
When is a atom a positive or negative based on the amount of electrons an atom has?