Why is "union" an exception to the "a/an" rule? [duplicate]

Usually when a word starts with a vowel, we will use "an" before it. But for union, it is "a union" not "an union." It is not explained in the previously mentioned a vs an why union is an exception. It explains how to know exceptions for h like hotels.

Also are there any exceptions for the opposite side, where there should be an "a" but is an "an" instead? (Unfortunately I have no example of a case like this.)

The a/an-rule is based on pronunciation, not on spelling. Though the word union is spelt beginning with a vowel, the u is pronounced "you":

/ˈjuː.ni.ən/

So, this is why it is accompanied by a rather than an and this is also the case for many other words starting with a vowel, have a look at these:

• a user
• a European

but:

• an ultimatum
• an orange

Note that there are words which start with an h and when that h is not pronounced, these words also go with an:

an honor

However, if this h is pronounced, then the article used is a:

• a hill
• a heathen

Here's a short but clear article that explains the usage of a/an: Articles: A versus An