 The pictured selenium isotope is used medically for diagnosis of pancreatic disorders. How many protons does an atom of this particular isotope contain? How many neutrons and how many electrons? So this one I think is pretty straightforward because the atomic number is given to us. We don't even have to look at the period of the table at all. So protons, neutrons. So how many protons does it have? 34. Why? Because the atomic number is the same as the number of protons. So number of neutrons, what would that be? How would we figure it out? Yeah, 75 minus 34 and that equals 41. So neutrons, right? The mass number equals the number of protons plus the number of neutrons. So in order to figure out the number of neutrons you take the mass number and subtract the number of protons. And last but not least, the number of electrons is 34. How do you know that? Because it's neutral because there is no charge. Everybody okay with that? Any questions on this? 78 on the periodic table. The mass of the periodic table. So here it's saying that, why is that different? It's got a different amount of neutrons. So this thing here, right, this is an isotope of this other selenium atom. How many protons does this selenium atom have? 34. So let's do this problem now that we're here. And I tripped over the thing. So protons, what do we say, 34? What's the number of neutrons? 40. Sorry, 44. And the number of electrons? 34. So this is just another what we call isotope of selenium. And in fact it's not, the one on the periodic table is not one in particular, it's the average of all of them. So when we say it says 78 on the periodic table, that's what's called the average atomic weight. And we'll get into that later today.