A single fluorine atom weighs 3.16x10^-23 grams. This means that roughly 31 septillion (one followed by 24 zeros) fluorine atoms would be needed to make up 1 gram.
As evidenced by this extraordinary number, atoms are extremely tiny — so tiny that a more convenient unit of measure is generally used in chemistry when talking about the mass or volume of a substance. This unit is called the mole, and the number of molecules in one mole is equal to 6.02x10^23. The atomic weights listed on the periodic table are given in grams per mole. To find the weight of a fluorine atom in grams, take the atomic weight of fluorine (18.99 grams per mole) and divide it by 6.02x10^23.