Colour and Electronegativity

Human beings are exquisitely sensitive to colour.

For the Ross model of the atom, I've chosen a particular spectrum.

• Metals have low electronegativity. All elements that present metallic characteristics are given cool colours, blues and purples.
• Non-metals have high electronegativity. The non-metals are represented by warm colours, yellows, oranges, and reds.
• The metalloids fall in a narrow band between 1.8 and 2.2. These are represented by greens. 
• Finally, the noble gases are represented by browns, because according to the Linus Pauling's original understanding, the noble gases were unable to form covalent bonds.

The overall spectrum looks like this:

One outcome of using colour in this way is to make it clear that hydrogen has an electronegativity, 2.1, that is very close to that of the metalloids. The non-metals all have e-neg greater than hydrogen; the metals' e-neg are all less than hydrogen's. 

This permits the fortuitous use of the colours of the famous Universal Indicator. Non-metals, of course, form acids. They do so, because, being to the right of the metalloids, they are able to pull electrons away from hydrogen. If hydrogen loses its only electron in this way, it becomes an acid - proton. 

Strictly speaking, the Lewis acid strength of each element is directly related to electronegativity. The greater the electronegativity, the greater the ability to remove the electron from a hydrogen atom. In fact other factors enter here, and the oxy-acid behaviours do not follow this system exactly in aqueous solutions. This is espcially true for oxygen and fluorine. 

Nevertheless, students can easily relate the bright red colour of Universal indicator to the strongest acids, suggesting a reasonable relationship.  

The metals tend to lose electrons to hydrogen, and thus to form bases. The strongest base in our limited table would be potassium. This corresponds to the deepest basic colour of the Universal indicator scale.

Neutral green solutions result when metalloids are mixed with water. Hydrogen is included here. In fact, the standard pH electrode is the hydrogen - platinum electrode. 

Take a look at the Ross Periodic Table to get a better idea off the overall effect.

Now even beginning high school students can work out the relationship between the core-charge, the valence configuration, the radius of the atom, and such phenomena as electronegativity and acid-base behaviour.

Where Would The Electron Go?

The first task using the Ross model that I give students is to make a decision on a selection of puzzles. These puzzles introduce the students to the concept of electronegativity, or its first cousin, electron affinity. 

Each puzzle consists of a representation of two atoms. 

Imagine that two different atoms are sitting close to each other. Imagine that you can drop a single electron into the space between the two atoms. Now imagine that the electron will move in such a way as to:

• occupy a vacancy in one of the valence shells, so that
• it is as close as possible
• to the largest core charge available

 Try it yourself, in the eight pairs of atoms in the diagram below. A short discussion follows the diagram. But hey! You are all teachers... So don't cheat!

1. The electron will be most strongly attracted to the place where it can get as close as possible to the strongest core charge possible. That is, it will occupy the only vacancy in the valence shell of the fluorine atom.
2. The two core charges, at 7+, are equally strong. The electron will move to the atom whose single vacancy is closest to the strong core charge. The electron is most strongly attracted to fluorine.
3. The two core charges, at 1+, are equally weak, in fact, the weakest possible core charge. Once again, the electron will occupy a vacancy in the atom with the smallest radius, that is, lithium. 
4. Neon has the smallest radius and the largest core charge. An electron in the neon atom's valence shell will be more strongly attracted than a valence electron in an oxygen atom. But neon's valence shell is already full! there is no more room there. The electron moves to occupy one of the two vacancies in the oxygen valence shell.
5. A sodium atom and a hydrogen atom are competing for the "free" electron. Both atoms have a weak 1+ core charge. The electron can get extremely close to the core of the hydrogen atom, and the attraction there will be much larger. The electron moves to the hydrogen atom.
6. Finally... carbon and phosphorus. Which atom will attract the electron the strongest? Will it be phosphorus, with its greater core charge? Or will it be carbon, with its smaller radius?  

It's interesting, isn't it, that most human beings (and certainly your students) experience the intense ambivalence of that last case. Why is that? My answer is that the powerful emotional tension of the ambivalence is caused by the very reasoning structures that students most often use. I really must write another blog about that.

This is the place to introduce the experimentally measured quantity electronegativity. That quantity is best introduced as one of the periodic trends, over at the periodic table.

The Three Salient Features of the Ross Model

Okay... We have transformed the Bohr-Rutherford model of the atom to the Ross model of the atom. This is a pedagogical model of the atom, designed specifically for learners.

The most important difference is that we have reduced the number of features from twenty-plus to only three salient features: the core charge, the valence configuration, and the radius of the valence.

More than a simple reduction of features, this atom organizes those features in a way that young people can understand. I have employed structures that match the sensory, psychological, linguistic, and motor factors that students use to "make sense of the world" that they perceive around them.

In a nutshell, it makes intuitive sense that:

• electrons are more attracted to "strong" core charges
• the closer an electron is to the core, the more strongly it is attracted
• an electron can only occupy an "empty" place in the valence shell  

Because of the pedagogical design, this representation of the atom makes intuitive sense to students. I do not doubt that you, too, will find this model intuitively easy to grasp.

Transformers: Bohr Model to Ross Model

To arrive at a pedagogical model, we will start with the Bohr-Rutherford atom. The B-R model presents a complete inventory of protons, neutrons and electrons, and it expresses the basic [nucleus + electron shells] structure of the atom. Let us begin with sodium and chlorine.

Start With the B-R Model

The B-R diagram of sodium consists of eighteen items, two of which are text expressions that must be considered together. This amount of detail far exceeds the mental capacities of teenagers, who can manage three to five things at one time.

The chlorine model is worse, with twenty four items. To a novice, it can hardly be distinguished from sodium at first glance.

There are few apparent causative relations in this structure. The only exception is that the total charge must equal zero (11 positive protons in nucleus require 11 negative electrons in electron shells). 

Beyond that, teachers (and students) have contented themselves for nearly a century with the notion that somehow the chlorine atom "wants" electrons, and the sodium atom "doesn't want" electrons. 

Remove Some Features and Collect Others Together

Eliminate the empty shells around the atom to get rid of some visual noise.

Group the nucleus plus the inner electrons together. These form a single entity, which we will call the core of the atom, or simply the core. This object resembles Ne neon in its electron configuration. It cannot accept electrons, because all of the shells in the Ne configuration are full. Furthermore, it contains more protons than neon itself, so it is even less likely to lose electrons than a neon atom.

The atomic core then is even more chemically inert than is neon itself.

We consider the valence electrons separately from the core. All of the valence electrons occupy a shell that is mostly outside the atomic core. We could even think of the valence electrons orbiting this small, positive, neon-like object.

Combine the Features of the Core Into One Simple Object

This is the step that bothers most chemistry teachers. "You can't do that!! I would mark that wrong!!" Relax... It's only a representation. There are many ways to think of the atomic core.

Perhaps the most familiar idea is that the sodium core is a nucleus with 11+ charge, shielded by 10- electrons. The effective nuclear charge is 1+. The chlorine core is a 17+ nuclear charge, shielded by 10- electrons, representing an effective nuclear charge of 7+.

Another way to think of the core of the sodium atom is that it is an Na + ion. The core of the chlorine atom as a Cl +7 ion. The matching number of valence electrons orbits around each central ion.

Finally, the diameter of the core is very much smaller than the valence. The core is a tightly packed ball of charge, unaffected by the chemical changes going on around it.

Represent the Valence Electron Shell at an Appropriate Radius

Chemists began measuring the radii of the atoms about a century ago. They are well known. Yet most representations of the atom do not include this important measurement.

The first thing the student sees is that the single valence electron of the sodium atom orbits very far from the small atomic core charge. Hmmm.. would that electron be strongly attracted to the core?

The valence electrons of the chlorine atom are much closer to the core.   Hmmm.. would those valence electrons be strongly attracted to the core? Is it possible that one additional electron could be attracted into the vacancy in the chlorine atom's valence shell?

Even from where I sit... I can see that you are intrigued.

What Are the Features of a "Pedagogical Model?"

Very simply, any pedagogical model applied to learning science should:

• Be easy to learn. It should require the least possible student effort, provide the greatest possible accuracy of student reproduction, and ensure the smallest possible set of structural student errors.
  
• Accommodate a wide variety of students. The ability, age, previous achievement and future learning of high school students spans a spectrum as wide as humanity itself. A pedagogical model should be within the intellectual grasp of most students within this spectrum. It should also enable those students to make progress, no matter where they begin on the spectrum.
  
• Be scientifically tenable. At the very least, it should have fewer errors than the B-R and Lewis models. It should match the largest possible set of explanations and predictions of more advanced theories. It should contradict more advanced models only at the very extremities of its application, and certainly not at its core.
  
• Advance every student’s capacity to explore. No matter where they begin on the spectrum, all students are driven to explore by their innate curiosity. A pedagogical model of the atom should enable students to explore their own questions about chemical behaviour.
  
• Advance students’ understanding of science. The epistemology of science is not easy to convey to novices. Naïve approaches to “methods of science” distort the scientific enterprise, and alienate many students from the study of science. A pedagogical model would invite students to participate in scientific investigation, and reward them with honest findings.
  
• Invite students to create knowledge. The scientific enterprise is about creating and verifying new knowledge. How can we initiate students into this epistemic activity? Memorizing dead science has not worked. “Discovery” science has not worked. A pedagogical model of the atom would support students as they create and express their own new knowledge of chemical behaviour.

Scientists use models to do scientists' work. Teachers need models to do teachers' work. 

So.. What's the Matter With Bohr?

For teachers, the Bohr model has been a godsend. Its regular, clockwork-like structure keeps kids occupied quietly for sixty minutes. Teachers can generate highly focused questions. Students can work out right-or-wrong answers.

But after the test, what good is it?

Honestly, teachers: can students look at a Bohr model of the sodium atom, and predict its soft, shiny, metallic appearance, its chemical activity with water, and its basic behaviour? No.

Be honest, now. Can students look at a Bohr model of the chlorine atom, and predict its non-metal appearance, its binary molecular structure, its reactivity, and its acidity? No.

The only thing the Bohr model is good for is memorization, and after-the-fact rationalization. Two things that kids hate in direct proportion to their ability to care.

Well.. the Bohr atom is only a representation. A picture. So..  like all good scientists, we modify the representation, and compare it to nature. (Yes. We have to consider our students as Nature.) In our case, we test the model to see if students who use it can make better predictions and explanations that those who don't use it.

In a later post, we'll see how the Bohr model is modified to become the Ross model.