Covalent Character in Ionic Bonds: Fajans' Rules

Transcript

0:00Introduction

Hello everyone! One small puzzle first. Take two salts: sodium chloride, and copper one chloride. In both, the cation has the same charge, plus one, and look at their sizes: ninety five and ninety six picometres. Almost twins! The anion is the same chloride ion too. Yet sodium chloride dissolves in water easily, while copper one chloride is insoluble in water. Same charge, same size, same anion. So why such a big difference? Strange, isn't it? Today we will crack it with three simple rules, called Fajans' rules. And in JEE, arranging compounds by covalent character comes up often. So, let's begin.

0:46Polarisation: the cation pulls the anion's cloud

So, look at the picture. In part a, on the left, the grey ball with a plus sign is the cation, and the pink sphere with a minus sign is the anion: no polarisation, purely ionic. Now part b, in the middle. The cation is small and highly charged, and the anion is large. So what will this positive cation do to the anion's negative electron cloud? It pulls it, yes, exactly! See the arrow at the top: electron cloud pulled, out of its round shape. This pulling of the anion's cloud by the cation is called polarisation. Okay? And what does it do to the bond? Part of the anion's cloud now sits between the two ions, a little like a shared pair. The double arrow on screen means so: a pulled cloud gives the ionic bond some covalent character. How much? The three lines at the right end of the picture decide it: Fajans' rules. Let's take them one by one. With me?

1:45Rule 1: a small, highly charged cation

Alright, rule one is about the cation, the one doing the pulling. Take two cations: a big one with charge plus one, and a small one with charge plus three. Which one pulls the anion's cloud harder? The small one with plus three, correct! More charge means a stronger pull, and a small size brings that charge close to the anion. So rule one: covalent character is greater when the cation is small and highly charged. Right? This pulling ability is called polarising power, and it goes roughly as charge over size. The wavy equals sign means roughly, and the slash means over. Bigger charge, smaller size, more polarising power. Got it? Now, a common doubt. Higher charge, stronger attraction, so the bond is more ionic, right? No, no, careful! That is the trap. Higher charge pulls the anion's cloud harder, so the bond becomes more covalent, not more ionic. Many students lose marks right here. Okay?

2:45Rule 2: a large, highly charged anion

Now rule two, about the anion, the one being pulled. Look at part b again: the anion there is large. Tell me, whose cloud is easier to pull out of shape, a small anion's or a large anion's? The large one, very good! In a large anion, the outer electrons are far from its own nucleus, so they are held loosely. And more negative charge means even more electrons in that cloud. So rule two: covalent character is greater when the anion is large and highly charged. With me? This ease of being distorted is called polarisability. So remember the pair: the cation has polarising power, and the anion has polarisability. Two different words, okay?

3:29Rule 3: the cation's shell matters

Now rule three. This one is special. Some cations have a pseudo-noble-gas shell. On screen: n minus one in brackets, then d with a small ten on top, then n s with a small zero on top. That means a full d subshell of ten electrons, and nothing left in the outer s. Examples: the copper ion C u plus, where the small plus on top is its charge, the silver ion A g plus, and the zinc ion Z n two plus. Clear? Now, these ten d electrons sit between the nucleus and the anion. Do they shield the nuclear charge well, or poorly? Poorly, yes! This d ten shell shields the nucleus poorly. So the anion feels more of the nuclear charge, and its cloud gets pulled harder. Okay? So such a cation polarises more than a cation of similar size and charge with a noble-gas shell, like sodium, N a plus, or potassium, K plus. Hold on to this rule. It solves our puzzle soon. Right?

4:31More covalent character means...

Alright, so the bond becomes more covalent. What changes? Ionic solids have high melting points, remember? So as covalent character grows, does the melting point go up, or down? Down, correct! More covalent character means a lower melting point. Got it? Next, solubility: less soluble in water, and more soluble in organic solvents. That is a clue for our puzzle! With me? Then, more colour, and more hydrolysis, which means reaction with water. Clear? And last, salts of oxo-anions, such as carbonates, decompose more easily on heating. The cation pulls the big anion's cloud and weakens its bonds. All of these come from one simple idea: the pulled cloud.

5:19Example 1: covalent character of NaCl, MgCl2, AlCl3

Okay? First example. Arrange the sodium ion N a plus, the magnesium ion M g two plus, and the aluminium ion, A l three plus, by polarising power. Then arrange their chlorides, N a C l, M g C l two and A l C l three, by covalent character. The small numbers below are subscripts, counting chlorine atoms. Pause the video and try it. Done? Okay, let's check. These three sit in one row of the periodic table. Along the row, the charge rises: plus one, plus two, plus three, and the size falls. So, which cation has the most polarising power? The aluminium ion, of course! Highest charge and smallest size. So, N a plus is less than M g two plus, which is less than A l three plus. The less than sign points to the smaller one. Okay? Now the chlorides. Aluminium has the highest charge, so the strongest attraction. So aluminium chloride is the most ionic, right? Oh no, careful! Remember the trap from rule one. The strongest polariser pulls the chloride cloud the most, so its chloride is the most covalent. Covalent character: N a C l, less than M g C l two, less than A l C l three. Same order as the polarising power. Got it?

6:43Example 2: why CuCl is more covalent than NaCl

Now, remember our puzzle from the start? Sodium chloride, and copper one chloride, C u C l, have cations of almost the same size: ninety five and ninety six picometres, written p m. Why is C u C l much more covalent, and insoluble in water? Pause the video and try it. Hint: use rule three. Done? Okay, let's check. Both cations have charge plus one, and almost the same size. So by rule one, both should pull the chloride cloud equally, right? No, no, wait! Charge and size are not the whole story. The sodium ion, N a plus, has a noble-gas shell, just like neon. Clear? But the copper ion, C u plus, has an eighteen electron shell, ending in three d ten, shown in brackets. And this shell shields the nuclear charge poorly. With me? So the chloride ion, C l minus, feels much more of copper's nuclear charge. The double arrow means so: C u plus polarises C l minus far more than N a plus does. Okay? The copper ion has a pseudo-noble-gas, d ten shell, so it polarises more. Then C u C l is much more covalent, and a more covalent compound is less soluble in water. Puzzle solved! Same size, same charge, but a very different shell. Beautiful, isn't it?

8:10Quick revision: small cation, large anion, d10 cation

So, quick revision. One: a small, highly charged cation gives more covalent character. Two: a large, highly charged anion gives more covalent character. Three: a cation with a d ten shell polarises more than one of similar size with a noble-gas shell. Now go and try the practice problems on this page. All the best!