Molybdenum (mp 2623 °C) has a higher melting point than yttrium (mp 1522 °C) or cadmium (mp 321 °C). Explain.

Video transcript

Welcome back, everyone. Let's take a look at our next problem. Explain why chromium has a higher melting point than scandium or zinc. A chromium has a higher melting point than scandium or zinc because chromium has more unpaired electrons and is de orbital B. Chromium has a higher melting point than Scandium or zinc because chromium has fewer unpaired electrons in its de orbital. C Chromium has a higher melting point than scandium or zinc because chromium has a smaller atomic radius or D chromium has a higher melting point than scandium or zinc because Chromium has a larger atomic radius. So we can see that the factors we want to focus on is a number of unpaired electrons or the size of the atomic radius. Both of these depend on the periodic qualities uh that we can see where the elements are on the periodic table. So our first step should be where are these elements on the periodic table? Well, they are all in the same row, all in row four. So we aren't going to expect huge differences in atomic radius since we're not adding extra shells, but they're in different columns. They are all in the D block, they all have electrons in their D orbitals, but in different places. So, Scandium has just one, the electron. Although that's somewhat misleading because as we know, there's exceptions to how those or electrons fill the D orbitals. So it's better to just say we'll call it number one in that row four D block, not number one in row four, excuse me, number one and in the D block specifically. So I'm going to put D block in parentheses because of course, we have group one A and two A before we get into that D block. So Scandium number one in the D block, chromium, number four in the D block. And zinc number 10 in the D block. So which of these factors unpaired electrons or atomic radius will have a bigger effect? Well, unpaired electrons definitely affect melting points because the more unpaired electrons, the more metallic bonds it can form. And therefore the stronger the more internal forces you have and the higher the melting point atomic radius does affect melting point, the smaller the atomic radius again, the stronger, the more stable that elect the more stable that element is going to be the higher the melting point. But again, we're all in the same row. So the difference in atomic radius here is going to be very slight. So the effect of this factor is going to be very small. So as an explanation for the difference in melting point we're going to go ahead and eliminate choices C and D. So let's focus on unpaired electrons in the D orbital. Well, we need to think about how are D orbitals organized? We have five D orbitals available and we don't pair electrons until all are full. So first you add a single electron to each one. So when we think about the number of unpaired electrons that's going to increase as we go from one, all the way up to five electrons. So as we go from left to right up until five electrons, our melting point increases with more impaired electrons. So let's imagine our row, we're going from one electron to the middle there. But now, as we add more than five electrons, we start putting them in pairs. So from 5 to 10, that melting point will be decreasing because now you have fewer and fewer unpaired electrons. Well, when we look at our locations, we see that scandium and zinc are at either end of that d block of elements. While chromium at number four is in the middle. So that's why chromium has this higher melting point. So we would select Chromium has a higher melting point because it has more unpaired electrons in its the orbital choice. B says that chromium has fewer unpaired electrons. Well, this is just wrong. So we crossed that out. So once again, Chromium has the higher melting point than scandium or zinc because Chromium has more unpaired electrons in its D orbital. And that is choice a see you in the next video.