Comparing Bond Angles

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Ammonia, N H three, and phosphine, P H three. Same group, both pyramidal, and both with one lone pair on the central atom. So both should have the same bond angle, right? But ammonia's angle is one hundred and seven degrees, and phosphine's is only ninety three point six. More than thirteen degrees smaller! Why? Today we find out, with four simple rules for comparing bond angles. In JEE, questions that ask you to arrange molecules by bond angle come up often. So, let's begin.

0:39Rule 1: more lone pairs, smaller angle

First, one idea from the start of this chapter. Electron pairs repel each other, but not equally. Lone pair with lone pair repels the most, then lone pair with bond pair, and bond pair with bond pair the least. The sign between them means greater than, okay? So lone pairs push hard, and they squeeze the bond pairs closer together. Everything today starts here. Now take three molecules: methane, C H four; ammonia, N H three; and water, H two O, where the small numbers below count the atoms. Each central atom has four electron pairs. But methane has zero lone pairs, ammonia has one, and water has two. So tell me, which one has the largest bond angle? Methane, yes, exactly! No lone pair to squeeze it, so one hundred and nine point five degrees. The small circle up top means degrees. Ammonia's one lone pair squeezes it to one hundred and seven. And water's two lone pairs squeeze it even more, to one hundred and four point five. So, rule one: more lone pairs, smaller angle. All three have four pairs, so the comparison is fair.

1:51Rule 2: same shape, central atom down a group

Clear? Now, back to our puzzle. Here are ammonia and phosphine, then arsine and stibine, with arsenic and antimony at the centre. Same pyramidal shape, one lone pair each, okay? And the angles: one hundred and seven, ninety three point six, ninety one point eight, and ninety one point three. Going down the group, the angle falls. Same story in the oxygen group. Water is one hundred and four point five, but H two S, hydrogen sulphide, is only ninety two. Right? So, same shape, same lone pairs. What changes when we go from nitrogen to phosphorus, or from oxygen to sulphur? Correct! The central atom gets larger, and less electronegative. Nitrogen is small and electronegative, so it pulls the bond pairs in close to itself. Crowded there, they repel each other strongly and keep the angle wide. Phosphorus is larger and less electronegative. Its bond pairs drift apart, farther out, and repel each other less. So the lone pair can squeeze them much more. With me? And one more reason. Phosphorus bonds with almost pure p orbitals; that is the italic p on screen. The p orbitals sit at right angles to each other, so the angle comes out close to ninety degrees. Ninety three point six, very close to ninety. Puzzle solved! Got it?

3:17Rule 3: electronegative outer atoms

Alright, rule three: same central atom, different outer atoms. Take ammonia, and swap every hydrogen for fluorine, the most electronegative element. N F three. Will the angle get bigger, or smaller? Smaller! N F three is one hundred and two degrees, less than ammonia's one hundred and seven. This sign, pointing the other way, means less than, okay? Many students expect the opposite, so remember this one. And the same with oxygen at the centre. O F two, at one hundred and three degrees, is less than water, at one hundred and four point five. Right? Here is the reason. Fluorine pulls the bond pairs towards itself, away from the central atom. Pulled away, the bond pairs repel each other less, and the lone pair squeezes them closer. Same idea as rule two. Clear? Now, C L two O, dichlorine monoxide. Chlorine is also more electronegative than hydrogen. So its angle must be smaller than water's, right? No, no, careful! It is one hundred and eleven degrees, bigger than water. Chlorine atoms are large, and they crowd each other. This steric repulsion pushes them apart. So large outer atoms can reverse rule three.

4:33Rule 4: a lone electron squeezes less than a pair

Okay? Now the last rule. N O two plus; the small plus up top is a positive charge. Nitrogen has no non-bonding electrons at all, so the ion is linear: one hundred and eighty degrees. N O two minus, with the small minus for a negative charge, has one full lone pair on nitrogen. It pushes the oxygens closer: one hundred and fifteen degrees. With me? Now, neutral N O two. It has just one lone electron on nitrogen, not a pair. Will its angle be above one hundred and fifteen, or below? Above, yes! One hundred and thirty four degrees. One single electron repels less than a full pair, so it squeezes the bonds less. Right? So the order: N O two plus, greater than N O two, greater than N O two minus. Nothing, then one electron, then a pair. Simple!

5:27Example 1: which is larger, the BF3 or NF3 bond angle

Alright, example one. Which has the larger bond angle: B F three, boron trifluoride, or N F three, nitrogen trifluoride? Pause the video, and try it first. Done? Okay, let's check. Both have one central atom with three fluorines. Same type of formula, so the same angle, right? Oh no, careful! Always count the pairs first. Use the quick count from the start of this chapter: add the central atom's valence electrons and the attached fluorines, then take half. The one over two on screen means half, okay? Boron: three valence electrons plus three fluorines, half of six, three pairs. Three fluorines take all three: no lone pair. Trigonal planar, one hundred and twenty degrees. Now nitrogen: five valence electrons plus three, half of eight, four pairs. Three fluorines, so one lone pair is left over. Pyramidal, and the angle is one hundred and two degrees. Even smaller than ammonia, thanks to rule three. With me? So, B F three has the larger angle, by a long way. Same formula type, but not the same angle. Always count the lone pairs first.

6:43Example 2: order NH4+, H3O+ and H2O by bond angle

Now a slightly harder one. Arrange H three O plus, water, and N H four plus by their HXH angle. The X just stands for the central atom, and the dashes are the bonds. Pause and try. Hint: count the lone pairs on each central atom. Done? Okay, let's check. N H four plus, the ammonium ion. Nitrogen has five valence electrons, plus four hydrogens, minus one for the positive charge. Half of eight: four pairs. Four hydrogens take all four, so no lone pair. One hundred and nine point five degrees, just like methane. Okay? Now H three O plus, the hydronium ion. Its central atom is oxygen, just like in water. So it has the same angle as water, right? No, no, wait! Count first. Oxygen has six valence electrons, plus three hydrogens, minus one for the charge: half of eight, four pairs. Three hydrogens, so only one lone pair. One lone pair, like ammonia: about one hundred and seven degrees. And water: six plus two, half of eight, four pairs again. Two hydrogens, so two lone pairs: one hundred and four point five. Clear? So, N H four plus, greater than H three O plus, greater than water. Zero, one and two lone pairs: that is rule one again, exactly like methane, ammonia and water. Easy, isn't it?

8:17Quick revision: lone pairs, group trend, outer atoms

So, quick revision. One: more lone pairs, smaller angle, and one lone electron squeezes less than a full pair. Two: for the same shape with lone pairs on the central atom, going down a group, the angle falls towards ninety degrees. Three: with lone pairs present, electronegative outer atoms shrink the angle, but large outer atoms can reverse it. Rules two and three work through the lone pairs; with no lone pair, the shape alone fixes the angle. Now go and try the practice problems on this page. You can do them. All the best!