You could try using 5 steps rather than 100 then a phrase like 'at least' or 'minumum available'.

By only changing your graph when one of the main chunks is completely full the user will only be able to deduce that there is somewhere between (in the example) 2 and 3 chunks of space available.

Your graph for either 41% of 59% would look like this. They would know they had at least 40% but not as much as 60%:







download bmml source &ndash; Wireframes created with Balsamiq Mockups

Where as one for 61% would look like this:







download bmml source

By breaking the graph down to the level of granularity that the system is able to report and no further the user would be given exactly the amount of information available. They couldn't be misled as you wouldn't be assuming and displaying a value as you are strongly implying with your example.

You could also number them 1-5 and add 'currently filling chunk n'

How to show a bar graph for an approximate percentage value in a given range?
I guess if we look at this problem from a "conjugate" perspective then the conjugate of a covalent bond is two elements with electrons lying around. On the other hand, with two electrostatically attracted things, the lack of a bond leaves two separate, charged particles. 

But that still doesn't answer the question; why are electrons less stable than ions? I guess in the case of an ion though there are still electrons but also a nucleus the stabilize the electrons. 
