Size, Density, Melting Point and Atomisation

Transcript

0:00Introduction

Hello everyone! Let me start with a strange pair of metals. Mercury and tungsten sit in the same row of the periodic table, and both are d block metals. Yet mercury is a liquid, while tungsten melts only at three thousand four hundred and ten degrees Celsius! Same row, same block. So why such a huge difference? Keep that question in your mind. Today we study size, density, melting point and atomisation of the transition elements. These trends come up often in JEE, so let's begin.

0:37Atomic radii across a series

Let us begin with size. As we move across a transition series, from left to right, the atomic radii fall at first, and then they stay nearly constant, okay? Fall first, then almost flat. Remember that shape. But wait. At every step the nuclear charge goes up by one, so the pull on the electrons keeps rising, right? Then why does the size stop shrinking? What is fighting that extra pull? The answer is the new electron itself. At each step the added electron goes into a d orbital, and this added d electron screens the extra nuclear charge well. More pull, but more screening too. They nearly cancel, so the radius hardly changes. Clear? Now let us go down a group. Zirconium is in the four d series, and hafnium sits just below it, in the five d series. Hafnium has one extra shell of electrons. So hafnium must be clearly bigger, right? No, and here is the surprise. In fact, the four d and five d radii are almost equal. Zirconium is one hundred and sixty picometres, and hafnium is one hundred and fifty nine picometres. The letters p m on screen stand for picometre, okay? Almost exactly the same size! The reason is the lanthanoid contraction, and we will crack it fully in Example two. With me?

2:03Density rises across a series

Now, let us look at density. Across a series, density rises. In the three d series it goes from scandium, at three point four, to copper, at eight point nine. The unit, g c m to the power minus three, is read grams per cubic centimetre, okay? So from scandium to copper, the density more than doubles. So tell me, why are transition metals denser than the s block metals of the same period? Three things work together. Their atoms are smaller, and they are heavier, with a higher atomic mass. And these atoms pack closely in the metallic lattice. More mass in less space means a higher density. Right? And the champions are in the five d series. Osmium and iridium are the densest, about twenty two point six grams per cubic centimetre. That is more than double copper.

2:56Melting point and enthalpy of atomisation

Got it? Now, back to our puzzle. First, let us define one term. The enthalpy of atomisation is the enthalpy change to break the metal completely into gaseous atoms, okay? The stronger the bonding between the metal atoms, the larger it is. Transition metals have high melting points, and high enthalpies of atomisation. So their metallic bonding must be strong. But why? It is the d electrons, exactly! Besides the outer s electrons, the unpaired d electrons also join the metallic bonding. More unpaired d electrons means more bonding, and a higher melting point. With me? So both values peak near the middle of a series. In the three d series, vanadium has the largest enthalpy of atomisation, five hundred and fifteen kilojoules per mole; k J mol to the power minus one is read kilojoules per mole. And tungsten, symbol W, near the middle of the five d series, melts at three thousand four hundred and ten degrees Celsius; the small raised circle means degrees. Now, manganese sits right in the middle of the three d series. So manganese should be the highest of all, right? Oh no, careful, it is the opposite! Manganese shows a dip, and in melting point, technetium below it dips too. Their stable d five set, with the five written small and raised, holds its electrons, so fewer of them join the metallic bonding. Remember this trap. And at the end of each series, zinc, cadmium and mercury, H g, have d ten, the ten written small and raised: a completely filled d set, with no unpaired d electrons. So their values are low, and mercury is even a liquid. There is our puzzle solved: tungsten, near the middle, has unpaired d electrons for bonding, and mercury, with d ten, has none. Clear?

4:53Second and third series: metal–metal bonds

One last trend, and it is short. The four d and five d metals have higher enthalpies of atomisation than the three d metals, okay? The reason is size. Their four d and five d orbitals are larger, more extended, so they overlap better between neighbouring atoms. Right? So the second and third series form metal metal bonds more often than the first. That one line reason is worth remembering. Got it?

5:19Example 1: atomisation enthalpy of Cr, Mn, Fe and Zn

Now, Example one. Among chromium, manganese, iron and zinc, which has the highest enthalpy of atomisation, and which the lowest? Use the trends we just learnt. Pause the video, and try it first. Done? Okay, let's check. The lowest is the easy one. Which of these four has no unpaired d electrons at all? Yes, it is zinc, of course! Zinc is d ten, completely filled, so only its two s electrons bond the atoms. So zinc is the lowest, okay? Now the highest. Manganese sits right between chromium and iron, in the middle. So manganese is the highest, right? No, no, careful, that is the trap we just saw! Manganese has the stable d five set, so it shows a dip. It cannot be the highest. With me? So the real contest is between chromium and iron, and here we need the actual values. Chromium is three hundred and ninety seven, manganese two hundred and eighty one, iron four hundred and sixteen, and zinc one hundred and twenty six, all in kilojoules per mole. So, iron is the highest, at four hundred and sixteen, and zinc is the lowest, at one hundred and twenty six. And look at manganese, two hundred and eighty one, lower than both its neighbours, chromium and iron. That is the d five dip, right there in the numbers!

6:49Example 2: why Zr and Hf have almost the same radius

Now, Example two, a little deeper. Let us go back to the two metals from the radii section. The radii of zirconium and hafnium are almost the same. Why? Pause the video, and think it through first. Done? Okay, let's check. First, what do we expect? Hafnium is in the five d series, with one more shell than zirconium, so it should be larger, okay? So something must be pulling hafnium in. Tell me, what comes between zirconium and hafnium in the periodic table? Yes, exactly, the fourteen lanthanoids! Before we reach hafnium, these fourteen elements fill the four f orbitals. Got it? And four f electrons screen poorly. So across the lanthanoids, the size goes down steadily. That is a steady contraction. Right? By the time we reach hafnium, this contraction cancels the increase we expected from the extra shell. So zirconium and hafnium end up almost the same size. And the answer for the exam is the lanthanoid contraction. Remember the surprise in the radii section? Now you know exactly why. Clear?

8:02Quick revision: radii, density and melting point trends

So, quick revision. First, radii fall, then stay nearly constant, and zirconium and hafnium are almost equal, thanks to the lanthanoid contraction. Second, density rises across a series, and osmium and iridium are the densest. And third, unpaired d electrons give high melting points, with a dip at d five and low values at d ten. Now go and try the two practice problems on this page. You will find them easy. All the best!