Moles of Diatomic Molecules
When we ask, for example, "how many moles of Iodine (or Oxygen or Hydrogen) is contained in 100g?" It ALWAYS refers to the molecule (e.g. I2, O2, H2 etc). Therefore, the number of 6.02 x 10^23 refers to the number of molecules, not atomes of Iodine and therefore you actually have 12.04 x 10^23 atomes of Iodine contained in one mole of molecular Iodine. However, when you look at the periodic system, they always refer to atomic mass not molecular. In case of Oxygen, the atomic mass of 16 g/mol refers to 1 mol of O, not O2 as molecular form. Further, when you want to know mass of the Oxygen (gas form, O2) then: 2 (number of atoms) x 16 g/mol = 32 g/mol - this is the mass of 1 mol of O2 (Oxygen-diatomic form - gas present in the atmosphere).