Solid lithium metal and diatomic fluorine gas react spontaneously to form a solid product. give the balanced chemical equation (including phases) that describes this reaction. indicate the phases using abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. express your answer as a chemical equation.

2 Li(s) + F2(g) ==> 2 LiF(s)  
First, just write the unbalanced reaction. Don't worry about the phases at this time. 
Li + F2 ==> LiF  
Now count the number of atoms of each element for each side of the unbalanced equation. 
Li: 1 and 1 
F: 2 and 1 
We're OK on lithium, but unbalanced on fluorine. We can balance that by having 2 LiF molecules on the right side. So: 
Li + F2 ==> 2 LiF  
Now count the atoms again 
Li: 1 and 2 
F: 2 and 2  
Now we're out of balance on the left hand side for lithium. Let's change its coefficient to 2 as well to correct that, getting: 
2 Li + F2 ==> 2 LiF  
And count the atoms again.
 Li: 2 and 2
 F: 2 and 2  And we're balanced. Now let's add the phase notations.
 2 Li(s) + F2(g) ==> 2 LiF(s)

The correct answer is a