A single atom is extremely small, typically measuring between 0.1 and 0.5 nanometers in diameter. The exact size depends on the element, with hydrogen being one of the smallest and larger atoms like cesium being among the biggest. Atoms are made up of a dense nucleus containing protons and neutrons, surrounded by a cloud of electrons that define their overall size. Because electrons exist in probability clouds rather than fixed orbits, the exact boundary of an atom is not sharply defined, but it