Everyone wants to be noble

              Science is gathering data and trying to make sense of it.  Just like detectives piecing together clues from a murder mystery, scientists take small hints about a world we cannot see and stitch a story that encompasses all the evidence we’ve gathered.  Our stories are models, which are different than facts.  The motive for murder is a model pulled together along with admissible evidence by detectives and tested in court.  The truth will never be known 100%, but the most probable way to explain what the police do know happened for sure becomes the working model for the lawyers.  Similarly, scientific models are pulled together by experimental evidence and tested in other experiments and in peer-reviewed work.  

                Let’s look at an example of how a model is built.  Facts: Amy spent her day at the park, then had a dinner of steak and mushrooms.  Following her dinner, she developed hives.  She would like a model to explain what caused the hives.  Experiments: The next week after her hives were gone, she went back to the park.  That night, she didn’t have any hives.  The next day, she ate a dinner of steak.  No hives.  Finally, she ate a dinner of mushrooms and that night developed hives.  Experimental Model: All of her experimental evidence suggests that she is allergic to mushrooms.  At this point, it is not proven that Amy is allergic to mushrooms, but it takes the experimental evidence and fits it with the known facts, thus creating a good working model for her: if she doesn’t want hives, then she should not eat mushrooms.  The model might be further amended as she gathers more evidence.  For instance, she might find out that only raw mushrooms give her hives while cooked mushrooms do not.  More evidence has refined her model.  This is very similar to how scientific models are created, reviewed, and updated.

                Models have been developed to explain what an atom looks like, how atoms behave, how atoms form bonds (actually, two models exist for this – some molecules are easily explained by one model and other molecules are easily explained by the other model; strange) and basically everything that happens at the level of atoms, molecules, cells, organs, etc.  

Our topic today deals with molecules and started with an observation about a special group of atoms.  If you look at the periodic table, the furthest right column is called the inert gases or the noble gases (Figure 66.1).  Scientists labeled them as such after they discovered that, no matter how they tried, these gases simply would not form molecules with anything else.  Carbon, oxygen and nitrogen readily jumped to form new molecules like carbon monoxide (CO), carbon dioxide (CO₂) or diatomic nitrogen (N₂), but helium, neon and argon couldn’t have been less interested.  They wanted to stay as neon (Ne), argon (Ar) and helium (He) only.  Why?  

Most people know that atoms are small nuclei comprised of neutrons and protons with electrons whirling around.  The sharing of electrons is also what causes two atoms to form a bond.  (I touched on this in my post What Does Water Look Like?)  Scientists needed a way to explain how these electrons are arranged and came up with a good working model, which I will explain very briefly here.

                Think of a school with two playgrounds.  The first playground is very close to the school building while the second one is a bit further away.  The first graders may only play on the close playground.  The fifth graders can play on either playground, but they mostly choose the further away one because what respectable fifth grader hangs out with six year olds?

                Okay, keep this in mind when we talk about electrons.  Let’s say an atom has six electrons.  Two of these six are like the first graders; they have to stay closest to the school building (nucleus).  The remaining four electrons have their choice of hanging close to the nucleus or being further away; most will be found further away. 

                The two electrons that must stay close to the nucleus are not involved in bonding with other atoms.  They are simply too close to the nucleus and are not interested.  It is the remaining four, which are predominantly found furthest away from the nucleus, that are involved in bonding.  Let’s call the electrons that are predominantly found furthest away the valence electrons.  An atom has room for eight valence electrons.   This particular atom only has four.  It would really really rather have eight.  How can our atom get four more electrons?  Why, by sharing electrons with other atoms!

                Check out Figure 66.2 and the water molecule I showed you in the What Does Water Look Like? post.  Each atom in a molecule or the neon alone has eight electrons around it.  The carbon or oxygen atoms alone have less than eight so they form a molecule such that, after sharing, each atom then has eight. 

                After investigating the electron counts in many different molecules and looking at the electron counts in the noble gases, scientists developed a model for why atoms form molecules.  It is summed up nicely as “Everyone wants to be noble.”  The inert gases have eight valence electrons already.  Atoms that do not have eight valence electrons form bonds (share electrons) with other atoms such that, once the sharing occurs, their valence shells will have eight electrons.  Everyone wants to have eight electrons.  The noble gases already have eight electrons so it is said that all other atoms want to achieve eight electrons to be like the noble gases.  Everyone wants to be noble.

* - Okay, one little thought: the water molecule in What Does Water Look Like? clearly shows that hydrogen only has two electrons around it.  Why doesn’t it have eight?  Well, because scientists realized that hydrogen and helium were different than every other atom.  Instead of wanting eight electrons, both of them are fine with two.  Every other atom follows the octet rule (I want eight electrons!); hydrogen and helium follow the doublet rule (two is cool).

                There are at least eight more posts I could write about electron configurations to fully explain what valence electrons truly are, where they hang out, what an orbital is, how orbitals change in an atom versus a molecule, etc.  However, the sentiment of this post remains true.  Rest assured, so much more could be said on this topic!

REFERENCES

Zumdahl, Steven S. “Chemical Principles, 4^th Edition” (2002) Houghton Mifflin Company, Boston, MA.