How to measure an atom?


Did you ever played with a drop of oil on the surface of the broth? Trying to stretch it to a bigger pond?
A certain Jean Perrin got a Nobel prize for this in 1926 – he measured atom dimensions.


„Dimensions” of the atoms can be obtained in many ways, but every answer is different. In gases, the atomic "dimension" is the distance, at which they start to push each other in a rather brutal way – their diameters are estimated with deviations from the perfect-gas equations, when it is impossible to pressurise more the gas* .

Similarly, in liquids, dimensions of the particles influence the “neighbourhood” effects, like the viscosity.

In crystals, atom diameters are defined as dimensions of elemantary cells, visible by the X-rays diffraction * *.

Atom diameters could be also evaluated from the density – if we knew how many atoms were contained in one portion, i.e. in 1 mole of the substance. But we need to know the advocate's Avogadro number, what is also difficult.

Finally, a drop of oil can be spread in the water and its and the diameter of the blot measured. In this way , if the blot is really big, say a mono-layer, knowing the volume of the oil drop one can simply calculate the "height” of the blot, i.e. the heigth of the molecules***. And the guy who did it first got the Nobel prize.


* it means with coefficients of the Van der Waals imperfect gas equation

** dimensions of atoms determined from crystal lattice constants are usually slightly bigger than those obtained using Van der Walls or viscosity equations

***The  “height” of the particles obtained in present experiment, with oleic acid is about 8-10
Å




© GK