An electron is unimaginably small, but it can be used as a handle to move relatively massive molecules and even macroscopic objects in an electric field.

How small? The English physicist John J. Thomson discovered the electron in 1897 and determined the ratio of its charge to its mass: 1.7 x 107 in cgs units. No way existed to measure the mass of an electron directly (nor does such a means exist today), so Thomson and others proceeded to make determinations of an electron's charge and thus permit calculation of its mass.

Even the earliest calculations of mass yielded tiny numbers. In mks units, the mass of the electron is now considered to be 9.11 x 10-31 kg (a little more than one two-thousandth of the mass of a proton). As for size, the electron can't be pinned down. Depending on circumstances, an electron acts like a particle or like a wave, or a little of both.