[Decessus00]

The following is one of the questions in my chemistry book:

Using the periodic table to guide you, predict the chemical formula and name of the compound formed by Ga and F.

This is a chapter 2 question which is about atoms, molecules and ions.  The answer is Gallium (III) fluoride which I understand how they got.  What I don't understand is why the answer couldn't also be Gallium (I) fluoride.  Since fluorine is always 1-, and Gallium isn't one of the elements listed as having a fixed charge, how am I supposed to know that Gallium is 3+? (I looked throughout the chapter and it didn't mention anywhere that Gallium is 3+ or use Gallium as an example problem with a 3+ charge)

[Borek]

There are some compounds of Ga(I), but these are rather exotic.

how am I supposed to know that Gallium is 3+?

Basically that's what the periodic table tells about the element.

Perhaps the most important thing about chemistry is that while it is good to know what trends do exist in periodic table, it is also good to not treat them too seriously and to remember, that there always some exceptions 

[Decessus00]

There are some compounds of Ga(I), but these are rather exotic.

how am I supposed to know that Gallium is 3+?

Basically that's what the periodic table tells about the element.

Perhaps the most important thing about chemistry is that while it is good to know what trends do exist in periodic table, it is also good to not treat them too seriously and to remember, that there always some exceptions 

I did notice on one of the online periodic tables that 3+ was listed as the most common form of Ga.  The periodic table in my book is rather limited as it only gives the symbol, atomic number, and mass number.  I appreciate your response, thank you.