Metallic Bonding

Transcript

0:00Introduction

Hello everyone! Okay, a small experiment to start. Hit a crystal of common salt with a hammer, and it shatters. Now hit a piece of aluminium with the same hammer. It does not shatter. It just flattens out. Both solids are held together by the pull between positive and negative charges, right? So why the difference? And one more puzzle. Sodium melts at just ninety eight degrees Celsius, but tungsten, also a metal, melts only at three thousand four hundred and ten! Today, one simple picture, the electron sea, will explain all of this. These ideas do come up in JEE, so let's get them clear. Let's begin.

0:42The electron-sea model

So, look at the figure on screen. This is a metal, seen from the inside. In a metal, every atom gives its valence electrons, its outer electrons, to the whole crystal. Now tell me: once an atom has given away its electrons, what is left behind? Positive ions, exactly! We call them kernels: the grey balls with the plus signs in the figure. They sit in fixed positions, in a regular pattern called a lattice. Okay? And the electrons? They are the small orange dots between the kernels. They belong to no single atom, but to the whole crystal, and they move freely all through it, like water in a sea. So we call it the electron sea. With me? Now the bond itself. The positive kernels are pulled towards the negative sea all around them, and this pull holds the whole crystal together. That is the metallic bond. Clear?

1:36What the mobile electrons explain

Alright, so what can this electron sea explain? First, put a voltage across a metal, and the mobile electrons drift along it. That is a current. So metals conduct electricity, even as solids, and the same moving electrons carry heat too. The double arrow on screen just means: therefore. Okay? Now a quick question. Common salt is an ionic solid. It is also full of charged particles, plus ions and minus ions. So why does solid salt not conduct electricity? Because its ions are locked in place, correct! They sit fixed in the crystal and cannot move. No moving charges, no current. In a metal, the electrons are free to move. That is the whole difference. Got it? Next, the shine of metals, called lustre. The free electrons at the surface absorb light of almost all visible wavelengths, and send it right back out. So the surface reflects light, and the metal shines. Right?

2:35Non-directional bond: layers can slide

Now, something special. In a covalent bond, the shared pair sits between two particular atoms, so the bond has a fixed direction. But here, the electron sea surrounds every kernel. So the metallic bond has no fixed direction. It is non-directional. So, think. If you push one layer of kernels sideways, past the layer below, does the bond break? No, it doesn't! After the slide, every kernel is still sitting in the same electron sea, still pulled from all sides. So the layers simply slide, and the metal stays in one piece. Clear? That is why metals are malleable: they can be beaten into sheets. And ductile: they can be drawn into wires, as the brackets on screen remind you. Our hammer puzzle? Half solved already. With me?

3:23What makes the metallic bond stronger?

Now the second puzzle: why do some metals melt so easily, and others hardly at all? It depends on how strong the metallic bond is. So, think. If each atom puts more electrons into the sea, does the bond get stronger or weaker? Stronger, exactly! More electrons in the sea means more negative charge holding the positive kernels, so a bigger pull. More valence electrons per atom, stronger metallic bond. Okay? Second, size. When the kernel is small, the electrons of the sea get closer to it, so the pull is stronger. Smaller kernels, stronger bond. Right? And a stronger bond shows up as a harder metal, with a higher melting point. The m p in brackets is just short for melting point. Clear? Now apply it. The alkali metals have one valence electron each. But going down the group, the kernels get bigger and bigger. So, does the melting point rise, or fall? It falls, correct! Oh no, careful here: many students think heavier atoms must melt higher. Same one electron each, but bigger kernels, so a weaker pull from the sea. So the alkali metals get softer, and melt lower, as we go down the group. Got it?

4:42Transition metals and mercury

Now the transition metals. Besides their s electrons, they have many unpaired d electrons, from the shell just inside the outer one. That is the n minus one in brackets on screen. And these d electrons join the sea too. So, what do you expect: soft and low-melting, or hard and high-melting? Hard and high-melting, yes! Tungsten, with the symbol W on screen, melts at three thousand four hundred and ten degrees Celsius. The small circle before the C means degrees. That is our second puzzle, solved. With me? But one metal breaks the pattern: mercury, written H g on screen. Its outermost electrons are a filled six s two pair; the little two up top means two electrons. Mercury holds this pair very tightly, and shares it very poorly with the sea. Okay? So mercury's metallic bonding is weak. That is why mercury is a liquid at room temperature, even though it is a metal. Clear?

5:40Example 1: why metals are malleable, ionic crystals brittle

Now, example one, and it is our hammer puzzle from the start. Why are metals malleable, while ionic crystals are brittle? Pause the video, and try it first. Done? Okay, let's check. In a metal, the bond is non-directional. So when the hammer hits, the layers slide, and the electron sea keeps holding the ions together. Nothing breaks. Okay? Now the ionic crystal, like salt. Inside it, plus ions and minus ions alternate, so each plus ion has minus ions next to it. Now shift one layer by one position. What sits next to each plus ion now? Another plus ion, yes! The shift brings like charges face to face, and like charges repel. So the layers push apart, and the crystal cracks. Brittle! That is exactly why our salt shattered. Right? So, the answer: non-directional bonding in metals. Perfect!

6:39Example 2: melting points of Na, Mg and Al

Now a harder one, example two. Arrange sodium, magnesium and aluminium, written N a, M g and A l on screen, by melting point and hardness, and explain. Hint: count how many electrons each atom gives to the sea. Pause the video, and try it. Done? Okay, let's check. Sodium gives one valence electron to the sea, magnesium gives two, and aluminium gives three. Okay? So the kernels are N a plus, M g two plus, and A l three plus; the small number and sign up top give the charge. Along this series, the kernels get smaller and more highly charged. Both factors point the same way. So which metal has the strongest metallic bond? Aluminium, correct! So the bond gets stronger from sodium, to magnesium, to aluminium. Each less than sign means the one on its left is weaker. Right? And the melting points agree: sodium, ninety eight degrees Celsius; magnesium, six hundred and fifty; aluminium, six hundred and sixty. Oh no, careful: magnesium and aluminium are close, only ten degrees apart, so do not mix them up. Still, more electrons in the sea means a stronger bond, and a higher melting point. Clear? So the answer: sodium, less than magnesium, less than aluminium, for both melting point and hardness. Very good!

8:02Quick revision: electron sea, properties, bond strength

So, quick revision. One: a metal is positive kernels in a sea of mobile valence electrons. Two: the mobile electrons give conductivity and lustre, and the non-directional bond makes metals malleable and ductile. Three: more electrons in the sea, and smaller kernels, mean a stronger bond and a higher melting point. Now go and try the practice problems on this page. All the best!