Chemistry: An Introduction to Electronegativity

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Now let's talk about a different experiment that is somewhat related to this notion of electron affinity and somewhat related to ionization energy as well, and that's an experiment that actually was done by Moseley in which he took a series of elements--and actually this was done with the Transition Metals--and he bombarded them with electrons. And what happened was an electron removed an electron in the atom of the sample he was looking at. So let's suppose these are iron atoms, for instance. He knocked out one of the electrons in the iron atom, and in fact, not in the electron--an electron deep within the iron core, so this might have been a 1s electron, for instance. That created a vacancy at the 1s orbital level. So it may come as no surprise to you that electrons that are at higher energy orbitals, that are paying a huge amount of rent for these miserable places, they see this vacancy in this 1s orbital right next to the nucleus and they're going to go flying into that orbital. Well, when that happens there's a release of energy. That energy again is released as a photon. If we understand the energy of that photon, that tells us this time not what the energy of this electron is or when it's over here, but what is the energy difference between the electron in this orbital and this orbital; in other words, in this state versus this state.
And indeed what Moseley found was there was a periodic trend, that as we went across the periodic table, that energy uniformly shifted to higher and higher values, indicating again that as we go across the periodic table the effective nuclear charge increases so the energy difference between the two orbitals increases. And as a result the frequency of the photon emitted uniformly increases as we go across the periodic table. So again, this is actually one of the earlier demonstrations of a periodic trend; that this absorption uniformly increases. And back when it was done, there was no notion of orbitals at all. Quantum mechanics had not been developed yet. But it was nonetheless an observation that showed this periodic behavior in the electronic properties of the atoms, as we increased molecular weight in this case. We now understand the reason for that, simply that the effective nuclear charge is increasing as we went across that period.
Finally let's talk about another related idea that in fact is not an experimental measurement. Up until now we've been talking about, you go in the lab and you measure these things. Okay, I want to talk about a concept that we're going to use a lot, that is not in fact a measurement but an idea, an idea developed by Pauling, in fact. This is an overall description of an atom's desire to take extra electron density or to give it up, all rolled into one idea. It's referred to as "electronegativity." Electronegativity right now loosely we'll define as the desire of an atom to take additional electron density, but this is in fact a combination of measurements having to do with ionization energy, electron affinity, and bond lengths even, which give us again some indication of an atom's general ability to want additional electron density from other atoms. This is all kind of rolled together to give us this notion of electronegativity.
And just quickly right now--we'll come back to this idea soon, but let's look at the general trend because we can make sense out of that with the periodic table at this stage. I'll bring in this key for us here, this color-coded key. Again, it's not important so much that you can read the elements; this is our overall periodic table. And what we find is the most electronegative elements are in the upper-right part of the periodic table: fluorine, chlorine, oxygen. These are the atoms that have the personality, if you will, of wanting most desperately additional electron density. And when we talk about electron affinity, that's going to be a big component for this. So it's kind of a weighing-out of electron affinity versus ionization energy.
On the other hand, at the bottom-left of the periodic table are the elements that have the least desire to take additional electron density. So these are the least electronegative elements. We also use the term "electropositive" to describe them, elements again that are much, much more willing to give up electrons than they are wanting to take additional electrons. We'll say a lot more about electronegativity as we go on and get into bonding, but at least we can start to introduce the general idea now. And the basic trend, once again, is that electronegativity increases as we go across the periodic table from left to right, and it decreases as we go from the top of the periodic table down. Once again, in general reflecting the overall increase in effective nuclear charge going across versus the increase in size and where the electrons are relative to the nucleus as we go down the periodic table.
So indeed, the beauty of quantum mechanics is that it, for the first time, explains these periodic trends and these observations. The planetary model could never hope to do this. And this is indeed the crowning jewel of quantum mechanics in that it describes all these properties so well. Once we know the personalities of these atoms, we can start to talk about how they come together to form bonds.
Electron Configurations and Periodicity
Periodicity
Introduction to Electronegativity Page [1 of 1]