The size of atoms is on the order of 10-10 meters. A special distance unit called the angstrom is given to this size range. Below is a chart of atomic radius vs atomic number. One angstrom is .1 nanometers so the scale goes from .5 - 2.5 angstroms.
Remember that the radius is half the diameter so the numbers of the graph should be doubled to get our "normal" view of size. The numbers on the vertical scale should then be 1, 2, 3, 4, and 5 angstroms. Notice the general increase in atomic size with increasing numbers of electron shells, and the relative decrease in size with increasing numbers of electron - proton pairs more strongly pulling the atom together. Actual atomic radii depend also on the other atoms in the molecule in which the atom is present. The above are true only for elemental molecules.
Atoms cannot be "seen or measured" in the ordinary sense of using light to look at them because the wavelength of light is 1000 times larger than the size of an atom. Electron microscopes must be used to "see" atoms because the wavelength of electrons is much smaller than light.
Below is a scanning tunneling
microscope image of the surface of a Mo18O52
crystal. The contrast is related to the positions of the molybdenum and
oxygen atoms on the surface.
