Re: The Problem of the Missing Constant

On Fri, 22 Dec 2006, hanson wrote:

"Timo A. Nieminen" <timo@xxxxxxxxxxxxxxxxx> writes:

Measure atomic mass, measure density of liquid; this should
give a reasonable estimate. The cool thing about atomic radii
is that they're almost all the same - the biggest atoms are only
about twice the radius of the smallest. [1]

[hanson]
Interesting! ... if so, then why is that so [1] when
their relative mass amount goes from 1 to ~250?

What does mass have to do with size of atoms? Since Rutherford we know that the nucleus, with essentially all of the mass, occupies an insignificant portion of the volume. The size depends on the electrons, not the mass of the atom. OK, the number of electrons depends on the number of protons, which generally increases with mass, but what effect would neutrons have on the size?

Anyway, just do it with a simple Bohr model. Find the radii of the innermost circular Bohr orbits for H and element 92 (ie, uranium). Consider a uranium nucleus with with 91 Bohr electrons orbiting it. On average, the 91 electrons screen 91 proton charges. Do you expect the behaviour of electron 92 to be that different from electron 1 in H?

The surprising thing is not that the heaviest atoms are only a little larger than the lightest, but that they not smaller.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

.

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