6 - Ionic Bonding
Since the ChemStudy curriculum was released in 1963, ionic bonding was always first.
You know the drill.....
- Memorize main block elements
- Memorize Lewis Dot diagrams
- Believe that fluorine "wants" this, sodium "wants" that.
- Na and F do opposite things to get the holy grail: the "full octet."
- Memorize list of "ionic valencies," some of which violate (4).
- Memorize math that looks like lowest common denominator. Or...
- Follow the Criss-Cross rule. And stop thinking.
We can do better than all of that. First, we must admit that
ionic bonding is a more complex process than covalent bonding.
The whole process is driven by energetics, not by atoms "wanting" anything.
1. Metals and Non-metals
Boron, silicon, and arsenic lie along the dividing line between the roughly 72 naturally occurring metals and 12 or so naturally occurring non-metals.
On the Ross Table, the metalloids are colored green. All of them have electronegativities close to 2. Note that hydrogen has the same colour / electronegativity as the metalloids.
The metals are given cool colors, blue to purple, to indicate electronegativity less than 2.
Non-metals are given the warm colors yellow, orange, and red to indicate electronegativity greater than 2.
The noble gases are colored brown, to indicate that they are inert.
All of the properties of metals (lustre, thermal and electrical conductivity, malleability, chemical activity) can be easily explained by the Ross model. Metals have small core charges and large radii, therefore they have loosely held valence electrons.
Non-metals have large core charges and small radii, therefore they have strongly held valence electrons. In fact, only these 12 or so elements are able to grab and hold other atoms' electrons. This explains virtually all of the common properties of the non-metals.
In other words: if an elements looks like a metal, it is also likely to readily lose valence electrons.
2. Another Look at Na and Cl
Consider sodium's single valence electron. It is only weakly attracted to its small distant atomic core.
Sodium's electron would be much more strongly attracted to chlorine's valence shell, where it would be much closer to a much greater core charge.
If you compare any two metal and non-metal atoms (except H) you will find the same pattern.
3. Metal:Non-metal Covalent Bonds Unlikely
Suppose that sodium and chlorine were to form a covalent bond, as we described in Lesson 5. The chlorine atom (core charge 7+, small radius) would strongly attract sodium's single valence electron into its one valence vacancy. But poor old sodium (core charge 1+, large radius) can only have the weakest of attraction for chlorine's unpaired electron.
The "bonding pair" would be strongly attracted to the chlorine core, but could hardly be considered "shared" with sodium's distant, weakly positive core.
The result would be a very long and fragile covalent bond between Cl and Na. Such a bond would be easily broken, even at low temperatures.
Any combination of metal and non-metal will follow this pattern (except H, whose electronegativity is close to that of the metalloids).
4. "Taking Custody"
Imagine that chlorine's attraction for sodium's weakly held valence electron is so strong that chlorine actually "takes custody" of sodium's electron. Three consequences follow:
1. No electron, no orbital. Sodium's "valence shell" disappears. Poof!
2. The sodium core is revealed: it's the sodium ion!!
3. Chlorine becomes the chloride ion.
The game changes. Because sodium's valence shell has disappeared, there is nothing holding these two particles apart.
5. Ions Get Close!
Any student can see that as soon as the ions form, they can get much closer under this arrangement than the covalent bond conjectured in (2) above.
Students familiar with electric charges can see that absolutely all of the charged particles are closer together under this arrangement.
This is true even when you represent the chlorine ion with a larger radius, as it actually happens.
The Ross model strongly supports teachers who use this explanation of ionic bonds in the introductory chemistry class. Try it! You won't have to talk about chlorine counting seven electrons and discovering a character flaw that can be satisfied by talking sodium into "donating" its only valence electron.
6. Ionic Crystal Lattice Formation
This is the driving force behind ionic bonding. According to the famous Born-Haber cycle for the reaction between metallic sodium and gaseous chlorine, the only exothermic process is the condensation of the ions into the crystal lattice. The "crystal lattice energy", the thermal energy released when the separate ions condense into a crystalline solid.
All of the other steps in the formation of an ionic compound, i.e. separation into single atoms, electron transfer, etc., were endothermic.
So... ionic bonding can be more honestly treated in the Ross model.
Download the sample exercise using the link below.
7. Is It Possible to Unlearn False Models?
Now we come to the most important feature of the Ross model: the feature that you cannot see.
Students don't have to unlearn anything in later science classes
Students do not have to "unlearn" an essentially fake account of bonding. Every feature of ionic and covalent bonding using the Ross model is a much closer approximation to the best theories of chemical bonding than the fake accounts in which atoms behave like covetous friends.
To learn an account of bonding requires modifying thousands, even millions, of neural connections, making them all just a tiny bit more connective. These same neural connections are also used in thousands of other mental models, so the protein and membrane adjustments that constitute learning are very subtle indeed.
Neurologically, it would be impossible to undo each and every neural adjustment that makes up a false account. Therefore, a student can never "unlearn" a false account.
Let's segue from neuroscience to a "method of science".
Why Is Science So Hard To Learn?
It seems that we can provide students with examples of science, language of science, science "experiments" and so on. But why is it so hard to teach a teenager how to "do science?"
We have some intriguing ideas in Lesson 7.
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.