Ion and eon are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary, but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted, or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Ion (pronounced “eye-awn”) is a noun. It’s a science word found in physics and chemistry. It means one or more atoms containing a positive or negative charge; this charge is based on a gain or loss of electrons.
Eon (pronounced “eee-awn”) is a noun. It means one billion years in astronomy. It can also be used more generally to mean many years, many centuries, or eras of time.
The following story uses both words correctly:
Ioan was trying to get through his chemical physics class homework. Several questions asked him to calculate information about ionic activity. Each question was taking hours to complete. Those hours grew to feel like eons of time had passed. He was developing a migraine.
You know, he thought, maybe I’ll just withdraw from this class and try something else.