"An atom 'is to' become a positive ion when it has more protons than electrons." Is it proper for "is to" to be in that sentence? If it is, would someone please tell me what does "is to" mean in that sentence? I wrote it, it's the answer for the question "How does an atom become a positive ion?".
It is an expression generally referred to as "a future obligation."
This guide is to provide you with all the information you need about our city.
You are to travel to their city tomorrow.
The lazy morons on this team are to blame for the failure.
In terms of strength, it is on par with "it is certain to". Both are stronger than "it needs to," "it will have to," "it would behoove it to," "he is determined to," and so forth.
It can be very useful in questions along the lines of:
Were you to tell me now that I'd be forced to listen to your wife's stories about her family, I'd think twice before agreeing to come along.
Your own example is poorly worded. Here's how I would put it:
If this lousy atom is ever to become anything even remotely resembling an ion, then you brainless troglodytes had better get off your fat asses and put in an honest day's work for a change.
Here is another exam I just found online, where "is to" replaces "will" though the time frame is less definite that with "will".
An English exam is to become compulsory for taxi drivers in London.