The VSEPR Model

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle first. Take a central atom with four electron pairs around it. Those four pairs always point to the corners of a tetrahedron. And yet, the molecule can come out tetrahedral, or pyramidal, or even bent! Same four pairs, three different shapes? Strange, right? Today we learn the VSEPR model, the simple idea behind the shapes of molecules. Questions on shapes come up often in JEE, so let's begin.

0:33Electron pairs stay as far apart as possible

So, here is the first idea. Keep an eye on the figure on the right; we will read it in a minute. The shape of a molecule depends on the number of electron pairs round the central atom. Now, some pairs make bonds, and some do not; those are called lone pairs. So tell me: do the lone pairs count too, or only the bond pairs? Both count, yes! Bonding pairs and non-bonding pairs, every pair round the central atom takes up space. Right? Now think. Every electron pair carries negative charge. So what will these pairs do to each other? They repel each other, exactly! So they arrange themselves as far apart as possible round the central atom. That is the heart of the VSEPR model. Clear? The figure shows the five basic arrangements, each with an example molecule in brackets; the small numbers below the letters count atoms. The thick line comes towards you, the dashed line goes away, and the small circle means degrees. Two pairs: linear, one hundred and eighty degrees, like B e C l two. Three: trigonal planar, one hundred and twenty, like B F three. Four: tetrahedral, one hundred and nine degrees twenty eight minutes, like C H four; the little dash means minutes. Five: trigonal bipyramidal, ninety and one hundred and twenty, like P C l five. Six: octahedral, ninety, like S F six. Learn these five well. With me?

2:08Multiple bonds and lone pairs

Alright, now a tricky one. Suppose the central atom makes a double bond. That is two pairs of electrons. So, does it count as two regions, or as one? Just one region! Double or triple, it counts as one. The extra electrons lie in the same region of space, around the same bond axis, so they do not point in a new direction. Okay? So, only the sigma pair decides the geometry. Sigma is the Greek letter on screen, like a small o with a tail. And notice: a multiple bond repels a little more than a single bond. Got it? Now, which pairs push the hardest? Two lone pairs, a lone pair and a bond pair, or two bond pairs? Lone pair with lone pair, yes, exactly! On screen, l p means lone pair, b p means bond pair, and the pointed sign means greater than. So: lone pair lone pair, greater than lone pair bond pair, greater than bond pair bond pair. The reason: a lone pair is held by one nucleus only, so it spreads out closer to the central atom and takes more room. Clear? So lone pairs push the bond pairs closer together: they squeeze the bond angles. This will explain many bond angles later. Right?

3:25The shape: look at the atoms only

Now the most important point, and the answer to our puzzle. When we name the shape of a molecule, we describe the positions of the atoms only, not of the lone pairs. The lone pairs push, but they are not part of the shape. Okay? So, four pairs and zero lone pairs: every corner holds an atom, and the shape is tetrahedral, the third drawing in the figure. Simple, isn't it? Now four pairs, but one of them is a lone pair. The pairs still point to the corners of a tetrahedron. So the shape is tetrahedral, right? No, no, careful! This is the trap. The electron pairs are arranged in a tetrahedron, yes. But one corner holds a lone pair, not an atom. Look at the atoms only, and they make a pyramid. So the shape is pyramidal. Got it? And with two lone pairs, two corners hold lone pairs, so only two atoms are left round the central atom. The shape is bent. Same four pairs, three shapes: tetrahedral, pyramidal, bent. Puzzle solved! Clear?

4:30Topper's trick: count the pairs quickly

Now, the topper's trick: count the pairs without drawing any structure. Pairs equals one half of, V plus M, minus c, plus a. Let's read each letter. Okay? Capital V is the number of valence electrons of the central atom. Right? Capital M is the number of monovalent atoms attached to it: hydrogen, and the halogens. Each of them adds one. With me? Now the charge. Suppose the species carries a positive charge. It has lost electrons. So, do we add that charge, or subtract it? Subtract it, correct! Small c, the positive charge, is subtracted, and small a, the negative charge, is added. Each negative charge is one extra electron. Clear? And now a trap. Oxygen and sulphur atoms attached by double bonds add nothing at all. They are not monovalent, so they do not go into M, okay? Students often forget this! Last step. Once you have the pairs, lone pairs equal pairs, minus the number of attached atoms. Every attached atom uses one pair, even with a double bond, because that is one region. Got it?

5:42Example 1: lone pairs in SF4, ClF3 and XeF4

Now let's use it. Example one. Find the number of electron pairs and lone pairs on the central atom of S F four, C l F three, and X e F four. Pause the video, and try it first. Done? Okay, let's check. S F four. Sulphur in the centre, six valence electrons. Four fluorines, each monovalent, so M is four, and no charge. Half of six plus four, half of ten: five pairs. Four atoms attached, so five minus four: one lone pair. Right? Next, C l F three, with chlorine in the centre. Tell me, what is V for chlorine? Seven, very good! Three fluorines, so M is three. Half of seven plus three, half of ten: again five pairs. But only three atoms are attached, so two lone pairs. With me? And X e F four. Xenon has eight valence electrons. Four fluorines: half of eight plus four, half of twelve, six pairs. Six minus four: two lone pairs. Notice: S F four and C l F three both have five pairs, but different lone pairs, so different shapes. More on that in the coming videos. Okay?

7:09Example 2: lone pairs in sulphate, XeO3 and SO2

Now a slightly harder one. Example two. Find the electron pairs on the central atom of S O four two minus, X e O three, and S O two. The two minus at the top right is the charge on the ion. Pause and try. Hint: watch the oxygens, and the charge. Alright, S O four two minus first. Sulphur has six valence electrons. And there are four oxygen atoms, so M is four, right? No, no, careful! Oxygen adds nothing, remember? So M is zero. But the charge is two minus, so small a is two. Half of six plus two: four pairs. Four O means four oxygens attached, so zero lone pairs. With M equal to four, you would get six pairs, completely wrong. Clear? Next, X e O three. Xenon, eight valence electrons. The three oxygens add nothing, and there is no charge. Half of eight: four pairs. Three oxygens attached, so one lone pair. Okay? And S O two. Sulphur, six. The oxygens add nothing. Half of six: three pairs. Two oxygens attached, so one lone pair. See how fast that was?

8:30Quick revision: repulsion order and the pair count

So, quick revision. Electron pairs spread as far apart as possible, and a multiple bond is just one region. Lone pair lone pair repulsion is the strongest, then lone pair bond pair, then bond pair bond pair. And the shape uses the atoms only. Finally, pairs equal one half of V plus M minus c plus a, and lone pairs equal pairs minus attached atoms. Now go and try the practice problems on this page. All the best!