An atom can vary in size. Neutrons, protons, etc. It depends on the element.
But even the biggest atoms (Atoms that compose Cesium, I presume.) cannot be seen underneath a microscope.
Mycoplasma (the smallest cell) can be seen underneath a powerful microscope whereas Cesium atoms cannot be.
I hereby conclude that an atom is smaller than cells. The answer differs between Chemistry and Biology but this is what I now believe.
