4 - From Bohr-Rutherford to the Ross Model

In this lesson, you will learn how to transform Bohr-Rutherford model into a pedagogical model - the Ross model.

The average adult can keep 5 - 7 simultaneous processes going on in their brains. The average 15-year-old can only keep 3 - 5 mental processes going.
As you read the next section, pay attention to the complexity of the B-R model. Does the number of details in the B-R model fit within the mental capacity of a typical teenager?  

The Original Bohr-Rutherford Model

BR - Ross 1Have you ever noticed the complexity of Bohr-Rutherford diagrams? Let's examine the learning task that faces your student.

The circle denoting the nucleus contains six bits of information.
Four additional circles represent the energy levels.
Attached to the energy levels are up to twenty-six electrons.
Then add the chemical symbol.
That's up to seventy-four different objects for the two atoms.

Each electron's importance is signified by its position.
That's potentially twenty-six positions on each atom.

100 bits of information. How many mental processes would that require? 

Even worse, all of this is drawn to wildly incorrect scales! The "nucleus" in the B-R model is ten thousand times too large. The various energy levels are at wrong by a factor of at least four. Why would we want our students to memorize something so wrong?    


Too Complex

With seventy-four objects, fifty-two positions, plus organizational processes, clearly the B-R model challenges the mental capacity of an average 15-year-old.

Why do traditional methods make students do that? The student cannot make one meaningful prediction of chemical properties from all of this detail. The only way a teenager can work with the B-R model is by reproducing the model on paper, and then interpreting or editing the model according to criteria that do not arise from the model itself. Even that is quite a task, and often takes several days of practice.

The Ross model can do better. 

Here's how we go from the B-R model to the Ross model:

Step One - Simplify

Gather the nucleus and all of the inner electrons (the core electrons) into one group (inside the colored circle). Combine all of the filled energy levels, leaving only the valence energy level on the outside.

We've reduced the number of objects and positions a little - but we need reduce more.







Step Two - Combine Core Electrons Plus Nucleus

B-R to Ross 3When combined together, the core electrons + nucleus become a single ball: the atomic core.

 In the case of sodium, 11 protons, 12 neutrons and 10 electrons have been combined to a single ball with a net charge of 1+. You can think of this atomic core as a neon atom, with one extra proton. Or you could think of it as a single sodium ion, around which the valence electron orbits. In any case, the atomic core a fraction of the size of the atom itself.

Chlorine's atomic core could be a neon atom with seven extra protons. It is extremely highly charged, and smaller than the sodium core. 

But does that little ball actually match the size of the inner atomic core?

Hit the button below to download a file that shows the size of the inner orbitals. Then re-consider whether the idea of an "atomic core" reflects orbital reality.

 download orbital table

Step Three - Draw the Valence Shell to Scale

B-R to Ross 4Every student and teacher knows about the atomic number Z.

But not one major introductory chemistry text pays attention to atomic radius in student thinking. Including the atomic radius is an important innovation in the Ross model. The Na and Cl atoms at left are drawn to scale.

So now the Ross model has been reduced to three salient features:

1. Core charge: bigger number = stronger attraction.
2. Valence configuration: number of electrons.
3. Radius: smaller radius = stronger attraction

    Students can use this systematic model to make hundreds of accurate predictions. 


    A Pedagogical Model

    We started with forty to fifty objects and positions in each Bohr-Rutherford atom, a number that added unnecessary complexity to the student's learning task.

    We are now down to three salient features: core charge, valence configuration, and radius. This is well within the intellectual capacity of your average 15-year-old student.

    This is a pedagogical model of the atom - a model designed so that students can learn about the chemical behavior of each element.

    As we will see in the next four lessons, these three salient features  enable students to use their intuitive reasoning to come to accurate conclusions about many chemical properties of the main block elements. 

    Meanwhile, think about the traditional way of teaching chemical bonding...

     "Chlorine wants one more electron to achieve a full octet. Sodium wants to donate one electron to achieve a full octet."

     And all of that.

     Please come with us to Lesson 5! The covalent bond can be very simply described using the Ross periodic table


    Lesson 1 - Three salient features of each atom across the periodic table.

    Lesson 2 - Students can predict electron attraction, with nearly no instruction!

    Lesson 3 - Take a closer look at the elements of the second row.

    Lesson 4 - Origins of the Ross model of the atom

    Lesson 5 - Covalent bonding

    Lesson 6 - Ionic Bonding

    Lesson 7 - A post-modern model of science.

    Lesson 8 - Learning with IntuitivScience. 

    In contrast, our recent publication Table Manners avoids this complexity, and provides the student with a dynamic model that works with the students' mental structures to make meaningful chemical predictions and accurate chemical explanations.

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