Bond Order, Stability and Magnetism

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. Take nitrogen, N two. Pull out one electron, and you get N two plus. Push in one extra electron, and you get N two minus. Both these ions have the same bond order: two and a half. Same bond order, so the same stability, right? Hmm, most students say yes. By the end of this video, you will know the real answer. Questions on bond order, stability and magnetism come up often in JEE, so let's begin.

0:34Bond order from the electron count

So, a quick recall from the last lesson. B O is short for bond order, and it equals one half, times N b minus N a, the line you see on screen. N with a small b below counts the electrons in bonding orbitals, and N with a small a below counts those in antibonding orbitals. Each antibonding electron cancels one bonding electron. Okay? Now tell me. What happens when the bonding count and the antibonding count are equal? Yes, exactly! Bond order zero. And bond order zero means no bond at all: the molecule does not exist. Right? Now, the first job in every question: count all the electrons. Take O two minus. The small two below counts the oxygen atoms, and the minus on top is the charge, okay? Oxygen has eight electrons, so eight plus eight, plus one extra for the minus charge: seventeen. And for a plus charge, you take one away. Got it?

1:33Bond order by total electron count

Alright, now let's put the second period molecules in one table, by total electrons. Six, L i two: bond order one. Eight, B e two: zero, so B e two does not exist. Ten, B two: one. Twelve, C two: two. See the pattern? From eight to twelve, every two electrons add one to the bond order. Right? So, what comes at fourteen electrons? Three! Correct! Fourteen electrons: N two, C O, C N minus and N O plus. All four have bond order three, the highest in the whole table. From eight to fourteen, each new electron goes into a bonding orbital. With me? Now, at fourteen, the bonding orbitals of the second shell are full. So where does the next electron go? Into antibonding orbitals, yes! So now the bond order falls. Sixteen, O two: two. Eighteen, F two: one. Twenty, N e two: zero, so N e two does not exist either. Clear? And the odd counts sit exactly halfway. Eleven: one point five. Thirteen: two point five. Fifteen: two point five. Seventeen: one point five. Nineteen: zero point five. One electron changes the bond order by one half. Okay? Now, a topper's trick. Thirteen and fifteen both give two point five. Twelve and sixteen both give two. Ten and eighteen both give one. Same distance from fourteen, same bond order! Learn one half of the table, and the other half comes free. But careful: this trick is for bond order only, not for magnetism. Let's see why.

3:21Magnetism: count the unpaired electrons

Now, magnetism. Really simple. If a species has unpaired electrons, it is paramagnetic. Okay? And if every single electron is paired, it is diamagnetic. Right? So, first rule. An odd total means one electron is always left alone. So exactly one unpaired electron, like fifteen, or seventeen. Got it? Now, O two has sixteen electrons, an even number. So all of them are paired, and O two is diamagnetic, right? Oh no, careful! This is the classic trap. Look at the diagram of O two; each little arrow is one electron. The last two go into the two pi star two p orbitals. Pi is the Greek letter, and the little star means antibonding. By Hund's rule, they sit one in each: two unpaired electrons! So O two is paramagnetic. And B two, with ten electrons, also has two unpaired, in its pi two p orbitals. With me? And twelve, fourteen and eighteen: every electron paired, so diamagnetic. Now see the trick from before: ten and eighteen have the same bond order, one. But ten has two unpaired electrons, and eighteen has none! Same bond order, different magnetism. Clear?

4:41Stability, bond length and bond energy

Now, stability. Higher bond order means more bonding electrons left over after the cancelling. So the molecule is more stable, and the atoms are held closer: a shorter bond. Okay? And a stronger bond needs more energy to break it. So higher bond order also means a larger bond energy. Right? Now, back to our puzzle. N two plus has thirteen electrons, and N two minus has fifteen. From our table, both have bond order two point five. On the right is the diagram of N two, with every electron paired. Got it? So, equal bond orders. Which one is more stable? Think: where does an electron leave from, and where does the new one go? Here is the key. To make N two plus, we take out the top electron of N two, from sigma two p z, a bonding orbital. Sigma is the Greek letter; in the diagram it is written sigma two p, with a small z below the p. To make N two minus, the extra electron must go into pi star two p, an antibonding orbital. So, for equal bond orders, more antibonding electrons, written e minus on screen, means less stable. Clear? So N two plus is more stable than N two minus. Puzzle solved! Same bond order does not always mean the same stability. This comes up often as a trick question. With me?

6:04Example 1: bond orders of O2+, O2, O2- and peroxide O2 2-

Okay, Example one. Give the bond orders of O two plus, O two, O two minus, and O two two minus. In O two two minus, the two minus on top means a charge of minus two, okay? Pause the video, and try it first. Done? Okay, let's check. Count the electrons first. O two has sixteen. One less for O two plus: fifteen. One more for O two minus: seventeen. Two more for O two two minus: eighteen. Right? Now just read the table. Fifteen gives two point five, sixteen gives two, seventeen gives one point five, and eighteen gives one. Each extra electron lowers the bond order by one half. Easy, isn't it? And from our stability rule: O two plus has the highest bond order, so the shortest and strongest bond of the four. Okay?

6:59Example 2: bond orders of NO, NO+, NO-, which is diamagnetic

Now a slightly harder one, with two different atoms. Arrange the bond orders of N O, N O plus and N O minus, and say which one is diamagnetic. Pause the video and try it. Hint: count the electrons first. Done? Okay, let's check. Nitrogen has seven electrons and oxygen has eight, so N O has fifteen. N O plus has one less, fourteen, and N O minus has one more, sixteen. Right? Now the table. Fifteen gives two point five, fourteen gives three, and sixteen gives two. The double arrow means so, and the greater than signs give the order: N O plus, then N O, then N O minus. Okay? Now magnetism. N O minus has sixteen electrons, an even number. So all paired, and diamagnetic, right? No, no, wait! Remember O two? Sixteen electrons means two unpaired electrons, exactly like O two. So N O minus is paramagnetic. N O, with fifteen, an odd number, has one unpaired electron. And N O plus, with fourteen, has none. Clear? So only N O plus is diamagnetic. See how the table did all the work? Count, read, and done!

8:20Quick revision: bond order, unpaired electrons, stability

So, quick revision. Count the electrons and read the bond order: three at fourteen, the highest, and zero at eight and at twenty. Odd count, one unpaired electron; ten and sixteen, two unpaired. And for equal bond orders, more antibonding electrons means less stable. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!