You cannot directly convert grams to atoms. First you must covert your grams to moles, then you can take the moles and covert to atoms. If you take your 878 grams of fluorine and then look at the atomic mass. You divide and find that 1 gram of fluorine is equal to 0.0525350025878 moles. Then you multiply that by your 878 grams. After you get that answer you can use Avagadro’s number, 6.022X10^23 to find the atoms. To get moles from atoms, divide number of atoms by 6.022 x 10^23. To get atoms from moles, multiply number of moles by 6.022 x 10^23.