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)

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.