Electronegativity

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. In a molecule of hydrogen chloride, hydrogen and chlorine share one pair of electrons. Shared means shared equally, fifty fifty, right? Well, not quite, because one atom pulls the pair harder, like a tug of war. So which atom wins, and by how much? Today we will put a number on it, with the idea of electronegativity. It comes up often in JEE, so let's begin.

0:31What electronegativity means

So, what exactly is electronegativity? It is the ability of an atom in a molecule to attract the shared electrons. Notice the words, in a molecule: an atom inside a bond, pulling on the pair it shares. Okay? Now, a question for you. Ionisation enthalpy is an energy of a lone gaseous atom, measured in kilojoules per mole. So tell me, is electronegativity also an energy we can measure like that? The answer is no, it is not. Electronegativity is not a measurable energy; it depends on the bonding situation, on what the atom is doing in that molecule. Right? It changes with the atom's oxidation state, hybridisation and neighbours. For example, carbon with s p hybridisation is more electronegative than carbon with s p three hybridisation; the three is the small raised number, and the greater than sign shows which is higher. Same atom, different bonding, different pull. Got it?

1:31The Pauling scale

Alright, now we need numbers, from the Pauling scale. Before I show it, tell me: which element pulls shared electrons the hardest of all? Yes, exactly, it is fluorine, right at the top at four point zero. Then oxygen, three point five; nitrogen and chlorine, both three point zero; carbon, two point five; hydrogen, two point one; and boron, two point zero. With me? And the lower values: silicon, one point eight; beryllium and aluminium, both one point five; lithium, one point zero; sodium, zero point nine; potassium, zero point eight; and caesium, zero point seven, the lowest in this list. Okay? Remember, Pauling values are pure numbers, with no unit at all. Right?

2:20Mulliken's formula

There is a second scale too, from Mulliken. The Greek letter chi stands for electronegativity, and the small subscript capital M means Mulliken's value. Chi sub M equals one half times the bracket, I E plus E A, and the e V on screen means electron volts. Okay? Here, I E is the ionisation energy, and E A is the electron affinity. So Mulliken's value is simply their average, in electron volts. Clear?

2:47Trends in the periodic table

Now let's find the trends from these numbers. Here is period two, from lithium to fluorine: one point zero, one point five, two point zero, two point five, three point zero, three point five, and four point zero. So what happens as we move from left to right? They go up at every single step! The double arrow on screen means therefore: electronegativity increases across a period. Got it? Now group one, going down: lithium one point zero, sodium zero point nine, potassium zero point eight, and caesium zero point seven. What do you see this time? That is correct, they fall: electronegativity decreases down a group. So fluorine, at the end of period two, tops the scale, and caesium, low in group one, is at the bottom of this list. Okay? And notice one more pattern. Metals like lithium and sodium have low values, and non-metals like oxygen and fluorine have high values.

3:48The difference decides the bond

Right? Now, for a bond, what really matters is the difference. We write delta chi; the capital Greek delta means difference, so delta chi is the difference in electronegativity of the two bonded atoms. With me? Let me ask you this. If one atom pulls far, far harder than the other, what kind of bond do you expect? Yes, exactly, an ionic bond! So a large difference, delta chi of about one point seven or more, makes a bond mostly ionic; the greater than sign with a wavy line under it means about this much, or more. But this is only a rough rule of thumb, not a sharp line. Okay? And Pauling gave a formula to estimate the ionic character. Ionic character equals one minus e to the power, minus delta chi squared over four. Here, e is the base of natural logarithms. Clear? Let us check it. If both atoms have the same electronegativity, delta chi is zero, and e to the power zero is one. So the ionic character is one minus one, which is zero: perfectly fair sharing. Right?

4:56Example 1: order of C, N, Si and P by electronegativity

Now, example one, a nice easy one. Arrange carbon, nitrogen, silicon and phosphorus in order of increasing electronegativity. Pause the video and try it first. Done? Okay, let's check. Silicon and phosphorus share period three, with phosphorus to the right, so silicon is less than phosphorus; the less than sign shows the order. In the same way, in period two, carbon is less than nitrogen. With me? Now the groups. Carbon sits just above silicon, and nitrogen just above phosphorus. Down a group the value decreases, so silicon is less than carbon, and phosphorus is less than nitrogen. Clear? So silicon is the lowest, and nitrogen is the highest. Only one pair is left: phosphorus against carbon. Phosphorus is to the right of carbon, so phosphorus must be more electronegative, right? Careful, because that is exactly the trap! Phosphorus is to the right, yes, but it is also one period lower, so the two trends pull in opposite directions. So check the values: silicon one point eight, phosphorus two point one, carbon two point five, and nitrogen three point zero. Carbon is higher than phosphorus. Okay? So the final order is silicon, less than phosphorus, less than carbon, less than nitrogen. Two trends and one look at the values, and the question is done. Got it?

6:28Example 2: ionic character of H-Cl and H-F from Pauling

Now example two, a little harder, and it answers our puzzle. Estimate the ionic character of the hydrogen chlorine and hydrogen fluorine bonds from Pauling's formula; the dash on screen stands for the bond. Pause the video and try it first. Done? Okay, let's check. For hydrogen and chlorine, delta chi is three point zero minus two point one, which equals zero point nine. Right? Now the power of e. Tell me, what is zero point nine squared, over four? The answer is zero point two zero two five. Then e to the power minus zero point two zero two five is about zero point eight two, and one minus zero point eight two is zero point one eight; the wavy equals sign means approximately. So the bond is about eighteen percent ionic. There is our puzzle solved: chlorine wins the tug of war, but the pair is still mostly shared. Okay? Now hydrogen and fluorine. Delta chi is four point zero minus two point one, which equals one point nine, a much bigger difference. So will this bond be more ionic or less ionic than the hydrogen chlorine bond? Yes, exactly, it is more ionic! One point nine squared, over four, is about zero point nine zero. Then e to the power minus zero point nine zero is about zero point four one, and one minus zero point four one is zero point five nine. So about fifty nine percent, more than half, because fluorine pulls much harder. Even so, hydrogen fluoride is still a covalent molecule, a very polar one, so remember that the one point seven rule is only a rough guide. And never forget to square delta chi; that is where students slip. Got it?

8:10Quick revision: trends, Pauling scale, ionic character

So, quick revision. Electronegativity is the pull of an atom on shared electrons, and Pauling values have no unit. It increases across a period and decreases down a group, with fluorine highest at four point zero. And Pauling's formula, one minus e to the power minus delta chi squared over four, estimates the ionic character; and, as a rough rule, a difference of about one point seven or more makes a bond mostly ionic. Now go and try the practice problems on this page. All the best!