 This one says copper has two naturally occurring isotopes, including copper 65, which is written up on the board. If you could, look at the periodic table and tell me whether the second isotope is either copper 63 or copper 66, so your choices are CU63 or CU66. So what's the other isotope of copper? So we have to look at the periodic table and how will we figure that out? Why are you 60 degree and 60 degree? So if we look at the periodic table it tells us that the mass of copper is 63.5 or 5, that's the average mass of a copper at. So if it's saying that this one is 65, one of the isotopes is 65, and there's only one more isotope, and the average mass is 63.5, which isotope does it have to be? 63. 63. Why is that? It's got to be something in between, because if it was 66, then you would expect this average mass to be in between 65 and 66. Does that make sense? So since it's in between 63 and 65, we can, with confidence, say that the other isotope is copper 63. Is everybody okay with that? On the side note, how many protons does this atom have here? 29. 29. Okay. And what about this one over here? 29. 29 as well. How did we know that? Because it's a copper atom.