Moles of Diatomic Molecules [duplicate]

Does "Mole of Iodine" or any other element that normally appears as diatomic molecules refer to a mole of single atoms ($$6.02\times 10^{23}$$ iodine atoms) or a mole of the molecules ($$6.02\times 10^{23}$$ molecules of $$I_2$$)?

EDIT: If there is no specification of moles of atoms or moles of molecules, what do I do? Do I default to one of them?

e.g. in this question

For the combustion of sucrose:

$$\ce{C12H22O11 + 12O2 -> 12CO2 + 11H2O}$$

there are 10.0 g of sucrose and 10.0 g of oxygen reacting. Which is the limiting reagent?

• It depends on context. Mole is simply a unit. Commented Jun 12, 2016 at 15:56
• Thanks for the link, I've edited the question to clarify. Commented Jun 12, 2016 at 16:50
• sigh If not specified otherwise it's about atoms and iodine is simply treated as element, not collection of molecules, as it doesn't have to be molecular. Commented Jun 12, 2016 at 16:57

2 Answers

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).

Good practice is to nip the problem in the bud by precisely defining what you are counting. Thus, properly, one mole of $\ce{I_2}$ molecules contains two moles of iodine atoms.