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
