The Actinoids

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. The f-block has two rows. In the top row, the lanthanoids, plus three is the characteristic oxidation state. Now drop down one row. Some of these elements go all the way up to plus seven! Both rows are filling f orbitals, so why does the lower row show so many more oxidation states? We will crack it before this video ends. This row is the actinoids; they come up often in JEE, so let's begin.

0:31Who are the actinoids?

So, who are the actinoids? They run from thorium, written T h, to lawrencium, written L r. Z is the atomic number: ninety for thorium, and one hundred and three for lawrencium. Count them, and you get fourteen elements. Okay? Now the general configuration. The square brackets with R n inside mean the radon core. Then comes five f, with a raised zero to fourteen, then six d, with a raised zero to one, and then seven s two. The raised numbers count the electrons in each subshell, and the dash means a range. Right? Now some real atoms. Thorium, right at the start, has no five f electron at all: it is six d two, seven s two. Uranium, written U, is five f three, six d one, seven s two. And plutonium, written P u, is five f six, seven s two, with no six d electron. Got it? Two more you must know. Americium, written A m, is five f seven, seven s two. Curium, written C m, is five f seven, six d one, seven s two. Keep these two in mind, because we need them in Example two. With me? Now, one big difference from the lanthanoids. All the actinoids are radioactive, and those after uranium are man-made. So let me ask you this: why would the chemistry of these elements be hard to study? Good, radioactivity is one part of the answer. On top of that, many of them have short half-lives, so they do not stay around for long, and most are made only in tiny amounts. Radioactivity and scarcity, that is the answer. Clear?

2:19Oxidation states: many more than the lanthanoids

Now, back to our puzzle. The actinoids show many oxidation states, going up to plus seven for neptunium, written N p, and for plutonium. That is far more than the lanthanoids manage. Okay? So here is the question. Why can an actinoid use so many of its electrons for bonding? Think about the energies of its outer levels. Yes, exactly, the five f, six d and seven s levels are close in energy. So electrons from all three levels can be used for bonding. Right? Now careful, because many students stop here and assume that five f behaves just like four f, tucked deep inside the atom. Is that true? Is five f as buried as four f? No, and this is the key point. The five f orbitals extend further out; they are less buried than the four f orbitals. Got it? So the five f electrons join in bonding. Close energies, plus five f electrons that can take part, and that is why the actinoids show so many oxidation states. Puzzle solved! With me? Now, which states matter most? Plus three is common, just like in the lanthanoids. Plus four is important for thorium, and plus six for uranium, found as U O two, two plus: there is a small two written low after the O, and a raised two plus for the charge. This is the uranyl ion. Clear?

3:46The actinoid contraction

Now, remember the lanthanoid contraction? Across the lanthanoids, the radii fall steadily, because four f electrons shield poorly. Okay? The actinoids show a contraction too, and it is greater from element to element than the lanthanoid contraction. So why should it be greater? Right, the five f electrons shield the nuclear charge even more poorly than four f electrons. So the size shrinks more from one element to the next. Clear?

4:17Actinoids versus lanthanoids

So let us put the two series side by side. What is the same? Plus three is common to both, and both show a contraction. Right? And the differences. The actinoids show more oxidation states, and they have a greater tendency to form complexes. Okay? Also, the actinoids are radioactive, and their five f electrons take part in bonding. Similarities and differences like these are asked often, so learn this list well. Got it?

4:45Example 1: highest oxidation states of U, Np and Pu

Now, Example one. Give the highest oxidation states of uranium, neptunium and plutonium. Pause the video and try it first. Done? Okay, let's check. Many students carry the lanthanoid habit and write plus three for all three elements. Is that right? No, that is the trap, so let us count for uranium. Uranium is five f three, six d one, seven s two. Three plus one plus two gives six electrons outside the radon core. Okay? Uranium is an early actinoid, and it can use all six of these electrons for bonding. So uranium goes up to plus six. Right? And neptunium and plutonium go even further, up to plus seven, just as we saw a minute ago. With me? So the answer is plus six, plus seven and plus seven. That is far beyond the plus three of the lanthanoids. Got it?

5:43Example 2: unpaired electrons in Am and Cm atoms

Now, Example two, a little harder. How many unpaired electrons are there in an americium atom, and in a curium atom? Pause the video and try it first. Done? Okay, let's check. Americium is five f seven, seven s two. An f subshell has seven orbitals, so the seven five f electrons go one in each orbital, all unpaired. Okay? And the two seven s electrons share one orbital, so they are paired. So americium has seven unpaired electrons. Right? Now curium. It is also five f seven, so the answer is seven again, right? Oh no, careful, there is one more electron hiding here. Curium is five f seven, six d one, seven s two, and that lone six d electron is unpaired too. So seven plus one gives eight unpaired electrons. With me? So the answer is seven for americium, and eight for curium. Forget the six d electron, and you lose the mark. Clear?

6:52Quick revision: configuration, up to +7, contraction

So, quick revision. First, the actinoids run from thorium to lawrencium, with the radon core, five f zero to fourteen, six d zero to one, seven s two, and all of them are radioactive. Second, they show many oxidation states, up to plus seven, because five f, six d and seven s are close in energy, and five f is less buried. And third, the actinoid contraction is greater, because five f electrons shield even more poorly. Now go and try the practice problem on this page. All the best!