We'll choose one of these water molecules at random (it doesn't make any difference which one!), and look at the bonding in a bit more detail - showing all the bonds around the oxygen.
Imagine for the moment that the 3+ charge is located entirely on the iron.
Can we change this to "imagine for a moment"? Would there be a significant change in meaning?
Am I right to assume that "for the moment" here means "for the short period of time while we are discussing the issue"? And "for a moment" would mean "for any undeterminate short period of time"?
I did an Ngram search for "imagine for the moment, imagine for a moment", and the latter option seems to be more widespread: