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.