Question: how small is an atom?

David Pyle answered on 24 Mar 2011:

It depends quite a lot on the element in question, and a bit on the local environment where the atom can be found. Most atoms range in size from about 100 to 200 picometres (100 – 200 x 1e-12 m, where 1e-12 means ten to the power of -12): you could line up 10 billion atoms side by side to make a line 1 mm long.

Sometimes you will find atomic radii measured in Angstroms. 1 Angstrom is 100 picometres, so most atoms have radii of 1 – 2 Angstroms. The smallest atom is He, with a radius around 30 picometres (though it depends how you calculate it); while caesium (Cs) is the largest of the periodic table, with a radius of about 300 picometres.

In comparison, the wavelength of visible light is about 5000 times larger than a typical atom.