Bond Length, Bond Energy and Heteronuclear Species

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. Take nitrogen gas, N two: two identical atoms. Now carbon monoxide: two different atoms, carbon and oxygen. And N O plus: two different atoms, and a charge as well. Three completely different species, right? Yet all three have exactly the same bond order, three, and not a single unpaired electron. Just a coincidence? By the end of this video you will know why, and you will also see how bond order decides bond length and bond energy. In JEE, questions on this come up often. So, let's begin.

0:40Isoelectronic: same count, same bond order

So, first a quick recall from earlier in this chapter. Bond order equals one half, times the bracket, N sub b minus N sub a. N sub b is the number of electrons in bonding orbitals, and N sub a is the number in antibonding orbitals. The little b and a below the N are just labels. Okay? Now look at the figure: the molecular orbital diagram of N two. The small two below the N means two nitrogen atoms. Seven plus seven, fourteen electrons. The Greek letters label the levels, and the arrows show the outer ten electrons; the inner four sit lower down and are not drawn. See, every arrow has a partner, one up and one down. The formula printed at the bottom of the figure counts only these outer electrons: eight bonding, two antibonding, half of six, three. Right? Now take C O, carbon monoxide. Carbon has six electrons, and oxygen has eight. So how many electrons in total? Fourteen, yes! Six plus eight, fourteen, exactly like N two. Now N O plus. The small plus sign up top is its charge. Nitrogen seven, oxygen eight, that makes fifteen. And the plus sign: do we add one electron, or take one away? Take one away, correct! Plus means one electron lost. So seven plus eight minus one: fourteen again! Oh no, careful here: many students add one for a plus charge. Got it? So, three species with the same number of electrons. We call them isoelectronic. Same count, so they fill the molecular orbitals the same way as N two, the ladder in the figure. The double arrow on screen is read as implies. So all three have bond order three, every electron paired: diamagnetic. Puzzle solved: not a coincidence at all! Clear?

2:37One electron more or less: NO and CN

Now, one electron more. Take neutral N O, with no charge. Seven plus eight, fifteen electrons. In the figure, fourteen electrons fill every level up to sigma two p z. Sigma is the Greek letter that looks like a small o with a tail. So where does the fifteenth electron go? Into the next empty level, yes: pi star two p, the empty pair of lines just above. Pi is the Greek letter that looks like a small table, and the little star means antibonding. With me? So, one more antibonding electron. N sub a goes up by one, and the bond order drops by a half: from three to two point five. And this electron sits alone in pi star. One unpaired electron, so N O is paramagnetic. Okay? Now the other way: one electron less. Neutral C N: carbon six, nitrogen seven, thirteen electrons. It is short of one bonding electron, so again half a bond is lost: bond order two point five. And thirteen is an odd number, so one electron is left unpaired. C N is paramagnetic too. Right?

3:48Heteronuclear molecules: which end binds?

Alright, now the long word in today's title: heteronuclear. It simply means a molecule of two different atoms, like C O, with carbon and oxygen. Quick question: in C O, which atom is more electronegative? Oxygen, correct! And here is the rule. In a heteronuclear molecule, the bonding molecular orbitals, M Os for short, lie closer to the orbitals of the more electronegative atom. So in C O, they lie closer to oxygen. Clear? Now, C O can bind to a metal using a lone pair. Through which end: carbon, or oxygen? Many students say oxygen, because the bonding orbitals sit closer to oxygen. Oh no, careful! The lone pair of C O sits mostly on carbon. So C O binds metals through carbon, not through oxygen. Got it?

4:41Bond order up: shorter, stronger bond

Now the big link of this video. Higher bond order means more net bonding electrons between the two nuclei. So, does the bond get longer, or shorter? Shorter, yes! And stronger too. On screen, the up arrow means increases, and the down arrow means decreases. So bond order up, bond length down, bond energy up. Okay? Let's see real numbers. N two, bond order three. Bond length: one hundred and ten p m, that is picometres. Bond energy: nine hundred and forty six kilojoules per mole, the k J mol to the minus one on screen. The minus one up top means per mole. Right? O two: bond order two. Bond length one hundred and twenty one picometres, longer. Bond energy four hundred and ninety eight, much smaller. Now F two has bond order one. Is its bond longer or shorter than in O two? Longer, exactly! One hundred and forty two picometres. And the bond energy is only one hundred and fifty eight. See the pattern: bond order three, two, one; the length goes up and the energy comes down. With me?

5:56Example 1: which species is isoelectronic with CO

Your turn now, example one. Which species is isoelectronic with C O: N two plus, O two plus, C N minus, or N O? Count the electrons. Pause the video, and try it first. Done? Okay, let's check. C O: six plus eight, fourteen. Now N two plus. N two has fourteen, the same as carbon monoxide. So N two plus matches too, right? No, no, wait! The plus takes one electron away: fourteen minus one, thirteen. And O two plus: O two has sixteen, minus one, fifteen. Neither one matches. Clear? C N minus: the small minus sign up top means one extra electron. So six plus seven, plus one, fourteen! And N O: seven plus eight, fifteen, one too many. With me? So the answer is C N minus. Fourteen electrons, just like C O, so bond order three, and diamagnetic. It joins our fourteen electron family. Perfect!

7:07Example 2: N-N bond lengths in N2, N2+ and N2-

Now a harder one, example two. Arrange the nitrogen to nitrogen bond lengths in N two, N two plus, and N two minus. Hint: use the figure. Pause the video, and try it. Done? Okay, let's check. N two: fourteen electrons, bond order three, as we saw. The highest bond order, so the shortest bond. Okay? N two plus: one electron fewer, thirteen. Which electron leaves? In the figure, the top filled level is sigma two p z, a bonding level. Lose a bonding electron, and the bond order falls by a half: two point five. Right? N two minus: one electron more, fifteen. It goes into pi star, antibonding, so the bond order again falls by a half: two point five. Now both ions have bond order two point five. So, the same bond length? No, no, careful! This is the trap. N two minus has an electron in an antibonding pi star orbital, and N two plus has none there. More antibonding electrons means less stable, so N two minus has the longer bond. Clear? So the order is: N two, less than N two plus, less than N two minus. Each less than sign means the bond on its left is shorter. Very good!

8:29Quick revision: 14 and 15 electron species, bond length

So, quick revision. Fourteen electrons, like N two, C O, C N minus and N O plus: bond order three, diamagnetic. Fifteen, like N O, or thirteen, like C N: bond order two point five, paramagnetic. Higher bond order: a shorter and stronger bond. And C O binds metals through carbon. Now go and try the practice problems on this page. All the best!