How can I get the the Avogadro constant?

edit

This is cool - however I spotted something strange: if I do "b=1 * units.amount_of_substance.mole", then "b.convert(units.amount_of_substance.elementary_entity" I get 6.02214179000000e23*elementary_entity while the official recommended value is 6.022141 2 9000000e23 (is it a typo somewhere?)

That's possible. If you can verify this with a standards organization link, open a Trac ticket to fix it and cc: whoever added the units package.

This has been fixed by Trac #12373: http://trac.sagemath.org/sage_trac/ticket/12373