What is the biggest atom? The smallest atom?
If by "biggest" and "smallest", you mean mass (which is a measure of how much matter is there), then the smallest is the hydrogen atom with one proton and one electron. Since electrons are about 2000 times less massive than protons (and neutrons), then the mass of an atom is mostly from the protons and neutrons. So the hydrogen atom "weighs" in as ONE. As you add more protons and neutrons, the mass increases. However, for very massive atoms, the force holding them together becomes unstable and they tend to break apart (a phenomenon known as radioactive decay). Very massive atoms such as nobelium and lawrencium have lifetimes of only a few seconds.
On the other hand, if you are speaking of size, then atoms are all about the same size whether it's a hydrogen atom (the simplest and least massive with one proton and one electron) or a lead atom (with 82 protons, 82 electrons and 125 neutrons). Atoms are composed of a nucleus (where the positively charged protons and uncharged neutrons reside) surrounded by a cloud of orbiting negatively charged electrons. An atom is about 10-8 centimeters in size (meaning that 100 million of them would fit side-by-side within one centimeter). The tightly packed nucleus is 100,000 times smaller than the electron cloud. You might think that as you add more protons (and thus more positive charge), the electrons would be attracted more strongly to the inner nucleus and hence the atom would shrink. In reality the electrons tend to screen each other somewhat from the inner positive charge and so the size stays about the same.
Carl Zorn, Detector Scientist (Other answers by Carl Zorn)