Electronegativity: Its Periodicity Across the Table

Electronegativity is often taught as one of those "authoritative" numbers. Someone in authority has measured it, and put it into text books, and teachers use those numbers to make tests more difficult.

I think that there is a better way to handle this. In this post over in The Ross Model of the Atom, I present a simple, intuitive model of the essence of electronegativity. With this approach, electronegativity is actually a solution to a problem that students encounter when they try to predict the behaviour of the atom.

As you probably know, there are several electronegativity systems that differ slightly. To keep confusion to a minimum, I've decided to keep the electronegativity to just two significant figures instead of three. In addition, for the second row of the table, I've chosen to use numbers from two different systems, so that the electronegativity numbers are very easy to memorize. This might seem a little "royal" of me, but remember... this is a pedagogical table, a table for learning. To require a student to learn a few small adjustments in later studies seems to me to be a good price to pay for getting the basic idea right in the first place.

So... In the second row, the electronegativity numbers are:

This series is very easy for beginning chemistry students to recall.

All of the atoms in the third row have valence shells that are just a little bit larger. Their valence electrons are a little farther away from the core charge, so all of the electronegativities are less than those for the atoms above them. This effect is greatest for the halogens ( chlorine is 1.0 less than fluorine ) and smallest for the alkali metals ( sodium is 0.1 less than lithium ). Comparing row three and row two elements, the difference increases monotonically across the row.

Now, memorizing all of the electronegativity numbers beyond row two is not the objective here. Making them appear logical and reasonable is the objective.

Core Charge: Its Periodicity Across the Table

Here are the main block elements of the periodic table.

First thing to note is that the core charge is periodic. Core charge is 1+    2+     3+    etc right up to 8+   and then it repeats. Furthermore, core charge is synchronous with the group numbers. Group 1 has core charge 1+, Group 15 has core charge 5+, and group 18 has core charge 8+.

 The next thing to notice is that the valence electron configuration is also periodic, in a way that is simpler than the B - R diagram. A student drawing the elements in Row 4 simply lists the core charges in order:
  1+   2+   3+   4+   5+   6+   7+   8+
The electron configuration follows exactly the same pattern.

Finally, the atomic radius is periodic. The radius in each row decreases monotonically, largest to smallest. Each subsequent row has a larger radius, because it is a higher valence shell. Apart from that, it follows the same pattern.

I find that even my grade 9 students (13 - 14 years of age) who normally struggle to learn can reproduce every aspect of the table above in 10 minutes. Except for the element names and symbols. I let the students pick those up gradually as they become familiar.

Now... is that easy to learn, or what? You are welcome to download this image for projection, or use in your own learning materials, as long as it is not published for profit. Just don't remove the name. I'm sure you understand.

The First Group: The Noble Gases

I love these guys. They were only "discovered" a century after the real nobles lost their heads. Lavoisier (and others) were social favourites because they were able to cook up nitrous oxide (laughing gas) for their noble friends. Now that was a noble gas. Robespierre had other ideas, however.

It is unfortunate that the scientific "noble gases" were so named because they were stand-offish, sniffy prigs. Chemically speaking. Like Robespierre. Politically speaking.

So... why are the noble gases "noble?" Why are they so famously inert?

For a very long time, adolescent chemistry learners have been taught by teachers (teachers!!) that the noble gases "want" a full octet. No teacher that I have met has been able to explain why an atom could have such a peculiar desire. Why not a full dozen? Or even a baker's dozen?

Think of it... first of all, the atom, with barely twenty parts, is capable of counting. So it counts up its electrons, discovers that it has only seven, and becomes "dissatisfied." Or it counts up its electrons, finds only two, and becomes discouraged. "What's the use?" says magnesium. "I might just as well give up now, and become an impoverished spinster. Or bachelor. Or whatever gender I am. A single one."

Think of it... an atom, with fewer moving parts than an amoeba, has emotions. And is able to formulate plans to acquire electrons from neighbours. Atoms with twenty moving parts having political ambitions! Think of it! And teachers (!!!!) have been responsible for propagating such silliness.

With the Ross diagram, no such fibbing is required. The first valence shell has the capacity for two electrons. No mystery there. A test tube has a capacity for 25 mL of water. You don't have to credit the test tube with intelligence, or emotions, or political aspirations. The rest of the valence shells have the capacity for eight electrons. Then they are full, and cannot hold any more.

The core charge of the noble gases is very high: 2+ for helium, and 8+ for neon, argon and the rest. The core charge does not get any stronger.

The radius of every noble gas is the smallest in its row. So the valence electrons are very, very close to a massive positive core charge.. they are very difficult to remove.

There you have it. Noble gases can't accept more electrons, because their valence shells are full. Noble gases can't lose any electrons, because their valence electrons are so strongly attracted to the core. Simple? Great.

And you don't have to tell your students that you have never lied in your life.

Why the Ross Periodic Table "Works"

It only makes sense to say that a table "works" if it can actually do work. The first thing to note about the Ross periodic table is that, compared to the Bohr model, the complexity has been reduced to just three salient items.

• Core charge - the "net" charge of the [ nucleus + inner electrons ]. You can see more here
• Valence configuration - basically the same as the Bohr and Lewis diagrams
• Radius - the distance at which the valence electrons "orbit" the core

This has weighty pedagogical implications. If you make the assumption that Jim Ross is an adult (as opposed to a stale teenager, well past the best-before date), you might attribute to my brain the capacity to work with five-to-seven different things at once. Your teenage students are capable of working with only three-to-five things at once.

The Ross model of the atom has only three salient features, so the adolescent learner must keep only three salient features in mind at one time. This is well within the capacity of the average teenage mind.

Not only that, but these three salient features actually "work" in cognitive patterns that correspond to students' everyday experience. That is... if your average adolescent student perceives these three salient features of a Ross atom in a "normal," "everyday experience" kind of way, that adolescent can make surprisingly accurate predictions of the chemical behaviour of the element. In that sense, the Ross table "works" in ways that other periodic tables do not.

I guess that I'll have to write another blog on the cognitive structures that I see teenagers using to explain novel experiences for which they have no other functioning models.

The cracks in the table... that's how the light gets in.

Recalling my first experience with the periodic table, reading my mother's high school chemistry texts, it seemed to me that the periodic table was a kind of "work of reference," a handy list of the elements, ordered into peculiar palisade of columns. It served mostly as a kind of arcane crossword puzzle.

So light bulbs are full of argon... where is argon on the periodic table?

or...

There really is a Krypton! The Superman comics are so cool!

To me, the structure of the table remained an exercise in authority, unquestionable as a stop sign.

Mr. Slavin was the first to take a hammer to the table. Mr. Slavin was my grade 9 electronics teacher, an inventor himself, who taught us the basic Bohr quantum description of the atom so that he could explain to us how semiconductors worked. In a few weeks, we were able to describe silicon doping, the structure and function of P-N diode junctions, N-P-N transistors, and amplifier circuits. Man... try that in Toronto today!

The light slowly began to appear through the first cracks in the odd ranks of elements. Mr. Slavin's vocabulary allowed some of us kids to begin to see patterns in valence shells of the Bohr atoms, related to columns of the table.

In my experience, that is precisely the place where most of my students began to perceive the structure underlying the periodic table. Check out the table in the header of the blog. You can see the familiar Bohr valence structure of the main block elements. But where are the rest of the details of the Bohr atom? You can find a discussion of that here. I'll continue here tomorrow.