Fe S is iron sulfide and has a molar mass 87.91 (Fe 55.845 S 32.06) 100g / (87.91g/mole) = # moles
1 Mole is 6.022 x 10^23 particles
100 / 87.91 x 6.022 x 10^23 = number of MOLECULES of FeS
EACH MOLECULE OF FeS has only one ATOM of Fe, so this is the number of Fe atoms.
Since FeS has TWO atoms for each MOLECULE:
The TOTAL number of ATOMS will be double this...... 100 / 87.91 x 6.022 x 10^23 x 2 = # atoms