Solid lithium and iodine gas are formed by the decomposition of solid lithium iodide .
write a balanced chemical equation for this reaction.

Lithium Iodide is
LiI

Lithium has a charge of 1+, and Iodide has a charge of 1-, thus this is the correct form. 

Remember Iodine is Diatomic when it is alone! So I_2

LiI (s) -> Li (s) + I_2 (g)

Is the formula you get when you simply write it out, but now we need to balance it. 

1 Lithium, 1 Iodide -> 1 Lithium + 2 Iodine

We need 2 Iodine on the left side, so we can add coefficients to the left to create that balance. 

2(1 Lithium, 1 Iodide) -> 1 Lithium + 2 Iodine
is the same as...
2LiI -> Li + I_2

Now we have to balance the Lithiums on the right so our final balanced equation is: 

2LiI -> 2Li + I_2

(Note that the _2 is a subscript of 2)

Electrical charges of ionic bonds