 All right, for those who didn't do too well on that test, let me re-explain some of this periodic trend stuff for you. The periodic trends really revolve around two key elements. They are Francium down here in the bottom left corner, and over in the top right cornerish area, Fluorine. Those are the two most important elements when you're trying to learn and work with these periodic trends. The periodic trends involve really three key concepts. We have ionization energy. This is the energy that has to be absorbed by an atom to free or liberate its valence electrons. Electron affinity, which is the amount of energy that is released by a non-metal when it gains an electron. When non-metals gain electrons, they get an opportunity to unload excess energy that they have, and this in turn makes them more stable, and electronegativity. This is the tendency of an atom to draw electrons towards it in a covalent bond. Those three big things. What makes these two elements so important is that of all the reactive elements on the periodic table, and of course when I say that, that means I exclude the noble gases, of all the reactive elements on the periodic table, Francium is the lowest in all these things, and Fluorine is the highest. We consider only the reactive elements. Francium is the lowest ionization energy, electron affinity and electronegativity. Whereas Fluorine is going to be the highest in all three of those things. What I do is I give you a set of elements, and I ask you to tell me which one is highest, which one is lowest in those three key areas. Let's say we're going to look at Sodium, Aluminum and Chlorine, those three elements. Now let's say I want to know the highest electron affinity. What I have to do is look at these three elements, Sodium, Aluminum and Chlorine, and see which one is closest to the highest element, and that would be Fluorine. Chlorine being right next door, is the closest to it, giving it the highest electron affinity. If I ask for the lowest of something, like the lowest ionization energy, I got to look to see which of those is closest to Francium. So again, Sodium, Aluminum, Chlorine, Sodium is the closest to Francium, so we would say it has the lowest ionization energy. Works the same way if those elements are in a group. So let's say this time I'm looking at Beryllium, Calcium and Beryllium, those three elements. Beryllium, Calcium, Beryllium. And I want to know which one has the highest electronegativity. Again, since I'm talking highest, I want to see which one is close to the highest one on the table. So again, Beryllium, Calcium, Beryllium, Beryllium is up here at the top of the table with fluorine, so it would be our answer, it's closest to the high one. If I ask for lowest, like lowest electron affinity, again I look at my three elements and I look to see which one is closest to the lowest, the Francium, and that would be Beryllium. Now the other trend that we work with on the periodic table is atomic radius, and atomic radius is the size of the atom. The definitions change here a little bit, this is no longer lowest, Francium is the largest, and this is no longer the highest, it's the smallest. And it's really that size that plays into the lower, higher thing. Because the Francium atom is the largest atom, it is the lowest ionization energy electron affinity electronegativity, and that has to do with the distance between the nucleus and the valence electrons. Because it's the largest atom, the valence electrons are furthest from the nucleus, and that's what causes those three things to be lowest. Over here where fluorine is the highest is that small size that makes it the highest. Again, small size that affects how far the electrons are from the nucleus. This time they're really close. That produces a strong attractive force between the electrons and the nucleus, making the ionization energy electron affinity electronegativity so high. Again, it would be one of those deals where you were given some elements. So let's say we're doing potassium, titanium, and arsenic. Potassium, titanium, arsenic. So one of the largest atoms. Well, the largest atom is the one of those closest to Francium. So again, it's potassium, titanium, arsenic, potassium is the closest, so it's the largest. But one of the smallest atom is the one that's closest to the fluorine. So potassium, titanium, arsenic, arsenic is closest to the fluorine. So it would be the smallest atom in that set. Final thing was knowing which ones shrink in size when they form ions, which one expand in size when they form ions. And that's all about knowing your metals and your non-metals. The ones I'm shading in here. These are your metalloids. To the left, you find the metals. To the right, you find the non-metals. What you got to know is these non-metals over here, they all expand, get larger when they ionize. And all these metals over here on this side of the table, they shrink when they ionize. So if I ask you for the ones that shrink, you're going to just pick the ones that are metals. If I ask which ones expand or get larger, you're going to tell me the non-metals. So if I ask copper, what does it do when it ionizes? You look through the periodic table and you find copper. To the left of the metalloids, so you know it's a metal, metal shrink. If I ask you about selenium, there's selenium. To the right of the metalloids, it's a non-metal. So it expands or gets larger when it makes an ion.