Atoms are composed of a massive, central nucleus surrounded by a
swarm of fast-moving electrons. The nucleus is made up of
protons and, in most cases, neutrons. Almost all of the mass
(more than 99%) of an atom is contained in the dense nucleus. An
atomic nucleus is much, much smaller than an atom. The cloud of
electrons that "orbit" the nucleus and define the "size" of an
atom is roughly 100,000 times as large as that atom's nucleus!
For example, a helium atom has a size of about 1 Ångström (0.1
nanometers or 10-10 meters), while its nucleus is only 1
femtometer (10-15 meters) in diameter. If you made a scale model
of an atom with a nucleus the size of a pea, the electrons would
zing around in a space larger than a major sports stadium! An
atom is mostly empty space. The number of protons in the nucleus
determines what type of element the atom is. The number of
protons is called the element's "atomic number". For example,
hydrogen has an atomic number of one, since all hydrogen atoms
have one proton in their