(Un)Happy Atoms

Question: Are the noble gases happy?

Answer #1: No because they exist as lonely atoms. Aren’t they also known as the inert gases? It sounds as if they have no passion. How could they be happy?

Answer #2: Yes. That is why they are one of the few substances that exist as atoms. They do not need to combine with other atoms. The atoms that are not noble gases are unhappy and want to combine with other atoms so they can be happy like the noble gases.

Answer #3: What does happiness have to do with atoms? You’re projecting irrelevant anthropomorphic ideas on to atoms.

Let’s rephrase the Question to something a classroom teacher might actually ask: Why are the noble gases the only elements in the periodic table that exist as stable individual atoms at room temperature and pressure?

Common student Answer: Because they follow the octet rule. They have a full shell of electrons and they are happy, um, I mean stable. They don’t want to react any further.

If you are a chemistry teacher, would this answer satisfy you on an exam? Perhaps, at an introductory level – maybe in a high school chemistry class or an introductory college course not aimed at chemistry majors. Let me point out three features of the response. (1) A rule is quoted. (2) A “full shell” of electrons is associated with “stability” but the latter is not explained. (3) Chemical reactivity is referred to as some sort of “motive”. Whether out of desire or necessity, some atoms want to react while others don’t want or need to.

The octet rule forms a bedrock in introductory chemistry textbooks. At a superficial level, it seems to magically provide an “answer” to those inscrutable rules of chemistry. If chemical transformation is about the combining and recombining of atoms, it can be used to “explain” how different elements in the periodic table “behave” differently: why some substances are solids, others are gases; why some substances conduct electricity while others are insulators. Why some are malleable, some are dense, and some are water-soluble.

We teach students to draw (Bohr-like) models of the atom to illustrate the octet rule. Here’s happy Neon

With eight electrons in its outermost shell, it is “full” and therefore unreactive. At the college level, an instructor or a textbook might try to connect the un-reactivity thus: “Noble gases have high ionization energies and zero electron affinities. Therefore, it is energetically very costly to remove an electron, and there is no energetic advantage to gaining an electron. The atoms are therefore energetically stable.”

Noble gases are less interesting because they are (for the most part) chemically inert. In a chemistry class, you want to get to the good stuff! All the other elements are not “stable” as atoms. They want to “combine” with other atoms. Here’s my tongue-in-cheek version.

If the noble gas electron configuration is the “happy” state, perhaps everything else wants to be like a noble gas to be “stable”. Notice how I’ve sneaked in a term of desire with a hint of anthropomorphism. Don’t we all want stability? Shouldn’t the noble gases also want to be stable (energetically)?

At this point, most textbooks introduce Ionic Bonding as a great way for metals and non-metals to achieve stability. And wow! Isn’t it amazing, ionic compounds (commonly known as salts) have all these unique physical properties that mesh sooooo well with our “lattice picture” of ionic compounds? If you’re paying attention I’ve just introduced two broad categories: metals and non-metals. (I’ve explained neither.) It’s a trick that chemistry instructors use by “appealing to the obvious” wherein we’ve now associated bulk macroscopic properties of different elemental substances in the periodic table to their atomic-level properties without much explanation.

Here’s how the story goes with cheeky pictures to illustrate the degree-of-happy. (Yes, I spent hours creating all the pictures in this blog post for the pure love of it!)

Alone, Sodium (Na) and Fluorine (F) are unhappy. But look! If Na transferred its valence electron to F, both of them would “achieve noble gas configuration”. See how they look like happy Neon. The ionic bond is formed by the attraction of the plus and minus ions! This works oh so well because metals want to give up electrons and non-metals want to receive electrons to be like the noble gas. Happiness all around!

But what if you have two non-metals? Both want to receive, neither wants to give. So what do they do? They share! Notice in my picture below how the two Fluorines look like happy Neons if you draw them sharing in just the right way. Now you have covalent bonds, which lead to covalent compounds and blah, blah, blah.

But what if you have two metals? Both want to give, neither wants to receive. Um, do they share? I don’t know. Let’s try this with two Sodiums. That doesn’t look like a happy noble gas. Is it stable? Dunno.

But what if they just gave up their electrons anyway into this mobile “sea” of electrons? It’s a “metallic” bond. Not only will they look like happy Neon, this will be excellent in explaining why metals conduct electricity. Bonus!

At this point we haven’t explained the nagging issue as to why the noble gases don’t bond even though the picture would look somewhat similar to the ionic bond you saw a moment ago, although one could easily hand-wave it away by referring to a later topic called “intermolecular forces”.

We’ve now explained how the three types of chemical bonding occur where we’ve blithely made use of the octet rule as a “driving force” for all these chemical reactions that involve combining atoms.

The problem is that the octet rule doesn’t “drive” anything. Energetics does. But it’s unclear what the energetic sources are, at least to the students. Hence, even if you as an instructor know that you are making a simplification, and even if you tell the students so explicitly, the happy anthropomorphic story outlined above sticks with the students like superglue. It seems to “work” so well (except when it doesn’t), and students being introduced to the complexity of chemistry latch on to a good heuristic. The octet rule is such a good heuristic, that even we instructors unwittingly use it to reinforce student misconceptions about the nature of chemical bonding. As a teacher, you might consider it a rule-of-thumb, but your students are likely using it as a rule. The thumb is lost somewhere along the way.

Below I’ve modified Figure 5 from Keith Taber’s article “A Common Core to Chemical Conceptions: Learners’ Conceptions of Chemical Change, Stability and Bonding” from Concepts of Matter in Science Education. This excellent article was also the motivation for this blog post and all my (un)happy pictures. The figure below illustrates some of the common misconceptions that stick with students as they consume the happy-atom story.

Honestly, I use something close to the happy story when teaching chemistry for non-science majors. It meshes well with textbooks and readings, and the heuristic “works” well for answering typical (or even standardized) exam questions. When I teach the class for science majors, I’m much more careful to stress energetics and what we mean by “stable”. This is done in detail for ionic compounds because it’s relatively “easy” to explain and supported by standard chemistry textbooks. Things are less straightforward for covalent compounds and metals. I do teach the octet rule and emphasize that it is a heuristic. But I strongly suspect, after reading the many studies in Concepts of Matter in Science Education, that my students interpret the octet rule differently. Actually, I know this to be the case because I can now clearly see the misconceptions that students make when they implicitly attempt to use the happy-atom story where it does not work. (It doesn’t matter whether you also teach orbital overlap, which is rather obtuse to the students.)

Is there an alternative way to approach chemical bonding instead of the happy-atom story? In the next post, I’ll outline an alternative idea (not mine) that I might try out in my General Chemistry class next semester.