Does diameter of a stable atom ever increase or decrease?

[ROOT BOY]

in a chunk of a pure element like iron-would the outside diameter of the the last electron shell ever naturally change from picosecond to picosecond?

 

[Simon Bridge]

Welcome to PF.
Technically an atom does not have an "outer diameter" in the same sense that a billiard ball has. usually a "radius" gets defined as the distance within which the electrons will be found some percentage of the time (like 98%).

The idea is to get a picture of the shape" of an atom, remembering that not all the shells are spherical.

So your question hinges on what you want to know for.
Generally electrons are more loosely bound if it is in an excited state (increasing that 98% probability distance)... some electrons promoted to higher energy orbitals. This can happen thermally or electromagnetically.

 

[mathman]

If the material is not at absolute zero (which it can't be) then electrons are constantly moving back and forth between higher levels and the ground state.