Silicon tetrachloride is reacted with very pure magnesium, producing silicon and magnesium chloride. (what would be the unbalanced equation? in other words i'm trying to find pure magnesium's formula.. would it be mg2? or just mg? ) i have sicl4 for silicon tetrachloride so far..
is it: sicl4 + mg = si + mgcl2 [unbalanced]?
balanced: sicl4 + 2mg = si + 2mgcl2 ? ?

Pure magnesium's formula would just be Mg because all elements except for 7 nonmetals are just left alone when they are by themselves in a formula. The 7 diatomic elements( means they have to have two of them without another element attached to it aka. a subscript two after it when it's by itself) are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. An easy way to remember the diatomic seven is that when looking at a periodic table if you trace over them from nitrogen over to fluorine and down to iodine all of those elements are diatomic + hydrogen. 

And your unbalanced and balanced equations are correct.

(sorry I went on a tangent with the diatomic rules hopefully it will help you in the future though) 

About 8 gallons of fuel will be used. (8.08)

Mg-26 and mg-24 are pair of isotopes. isotopes are defined as atoms of the same element having same atomic number but different mass number. i hope that was .