The question of what is the
longest English word is an
old chestnut, and one with a variety of possible answers.
As above, there are many (perhaps infinitely many) exceptionally long words possible as chemical names, by combining any number of prefixes and/or suffixes to give a descriptive name for a complex molecule.
The longest word to make it into any standard dictionary is pneumonoultramicroscopicsilicovolcanoconiosis (also spelled pneumonoultramicroscopicsilicovolcanokoniosis. This is supposedly name for a form of a miners' disease (silicosis) caused by extremely small volcanic dust.
However, this word is believed to have been coined by word puzzlers, and still reeks of technical jargon.
Most of the other top candidates are also of doubtful origin, though. There are others that top out around 28 letters, such as floccinaucinihlipilification (coined from 4 Latin words meaning "nothing"), and antidisestablishmentarianism (a coinage meaning a philosophy of opposition to breaking the existing establishment). Electrophotomicrographically is perhaps legitimate.