I might be wrong BUT,
1 mole of a substance has 6.02 x 10^ atoms
And I think the mass of one mole of a substance is equal to the Mr of the compound/element. So in this case 23g
So if 1 mole is 23 g, and you want to find the mass of a single atom,
23/(6.02x10^23) gives the mass of 1 atom.
But again, I might be wrong. I'm hoping someone might be able to confirm or correct this because I'm interested to know too!