 Isotopes were a bit of an issue, so today's bell work is on the isotopes. When you have an isotope, it'll be written in isotope notation. You have the symbol of the elements, this one's helium. The number on the top is the mass number for this particular isotope. This number does not have to match the periodic table. The number on the bottom is the atomic number, number of protons. This must match what's on the periodic table. If you were to look up helium, again, mass is 4. The number we have here does not have to match the periodic table. It just happens to, in this case, the atomic numbers on the bottom. That one must match the periodic table. The only thing special about these is the neutrons. These are neutral atoms, so none of these have a charge. These are not ions. There's nothing in the upper right-hand corner. Everything over here, I don't know why I have that on there. I should have taken that off. I'm sure I'm going to tell you that when we get to class. I copied and pasted. I forgot to take that column out. None of these have a charge, so they all work out like the normal neutral atom, except for those neutrons. We're still going to use the mass number minus the atomic number. What we're going to do here is use this mass. Not the mass off the periodic table. We're starting off with chlorine. The mass is 36, and the atomic number is 17. Protons will still be equal to the atomic number. That's the number on the bottom. When we get to neutrons, we don't use the periodic table. See on the periodic table here, it says the mass of chlorine is 35.45. Because that 5 is followed by a 4, we leave it alone. And we say the mass is 35. What this symbol is telling me is I have to use a mass of 36. I don't want to use the mass that's on the periodic table. So instead of having 18 neutrons, like the periodic table says it should, this one has 19. And again, it's still just mass number minus atomic number. But we're going to use that mass, 36 minus 17, to give us that number. These are neutral atoms. There is no charge over here. So the number of protons and electrons will be the same. Energy level is still period number. Chlorine is in period three. So it has three energy levels. Valence electrons still based on group. Chlorine is in group 17. Drop the one that has seven valence electrons. So next one up is oxygen with a mass of 15. Again, the protons are still atomic number. Atomic number is still the number on the bottom. It has eight. Neutrons are still mass number minus atomic number. But we're going to use the mass of 15. The periodic table says oxygen is supposed to have a mass of 16. So it's supposed to have eight neutrons. But this is not normal oxygen. It has a mass of 15. So we do 15 minus eight. And we see that we only have seven neutrons in this one. Protons and electrons are still the same because this is a neutral atom. Energy level is still period number. Oxygen is in period two. And valence electrons are still based on the group number. Oxygen is 16. Drop the one we have six. This is not normal carbon. This is carbon 13. Protons are still the atomic number. Atomic number is what's on the bottom. There are six protons. For neutrons, it's mass number minus atomic number. The mass number for this carbon is 13. So it's 13 minus six. There are seven neutrons in this one. And the protons and electrons are still the same because there is no charge. I can do all of this just looking at that symbol. I don't even have to look at the periodic table. Because again, the mass numbers on the top, the atomic numbers on the bottom. Those are the two pieces of information I would get off the table for those first three columns. I don't really need the table until I get over here. Period number still tells us the number of energy levels. There's carbon. It's in period two. And the number of valence electrons is still based on the group number. There's carbon. It's in 14. Drop the one. It's got four valence electrons.