One of my oldest memories is young me asking one of my parents what stuff is made of. Apparently I was always curious and a bit annoying. The answer was: “The four elements: water, fire, earth and air”. I was obviously not satisfied. Just look at a plant, or yourself and you can tell, there’s not much soil, flames or wind (unless it’s brussels sprouts day). Later in middle school, when I learned about the periodic table, and the elements that our world is actually made of. Having all these different atoms, forming molecules that then make up matter, this made sense to me. And maybe it was at that point, that I started getting interested in science. On the note, what matter in our universe is made of, I want to tell you what makes a molecule, and what doesn’t, so that we lay down the basics for other posts. And I want to convince that it’s quite satisfying to look at our world through the eyes of a chemist.
Zoom in a lot…
If we could take a magnifying glass and zoom into any structure, we would at some point see a molecule (here is why we can’t actually do this in practice). You would then see that these molecules are formed by atoms. And if you zoomed in further, you’d see that the atoms themselves are made up of electron clouds around a nucleus. In the nucleus, we find protons and neutrons that themselves are made… Stop! That’s where the world of a chemist ends. We don’t really care what’s in the nucleus. We concern ourselves with electrons in the outer shells of atoms. And that’s exactly where the magic happens. A chemical bond is formed, when two electrons from different atoms decide that they’ve had enough of staying alone in their orbitals while all other electrons have a partner. “I’m gonna get cozy with this electron from that carbon atom over there, so I can get closer to that handsome piece of nucleus!” Off they go, happily pairing and forming a chemical bond between two atoms. That chemical bond is what makes a molecule.
…and zoom out a little bit
Zooming a bit back out, you’d see not just one molecule, but many of them. All the electrons remain in the molecules. Some very close to the nuclei of each atom, and some forming chemical bonds between atoms, holding them together. There are also forces acting between those molecules, either attractive forces, like in the case of water, or repulsive forces, pushing them away from each other, like when you add a drop of oil into water. Oil and water don’t like each other, so the oil stays on the surface, where it’s exposed to the water the least. These attractive and repulsive forces are numerous and also a bit complex, so I don’t want to go into details here (keywords for the curios: hydrogen bond, Van der Waals force, London dispersion, ionic bond, Coulomb forces). The important point here is, that in all these cases, the electrons stay where they are and don’t leave the molecule (there are of course exceptions, as to everything in science).
What else doesn’t form molecules? Here are a few examples:
- Some atoms rather stay alone forever. We know them as noble gases, like helium, argon, neon etc. EVERY other element can basically not be found without being bonded to another atom. Sometimes it is just two of the same kind, like for molecular oxygen, nitrogen, hydrogen and some more.
- Metals don’t really form molecules, which means they don’t share a specific electron pair, but every metal atom in your fork gives one electron for the community, and so every individual atom is happy. This very socialist attitude leads to a special property: With all the freely moving and unbound electrons, they can transport other electrons through the material, meaning they are conducting.
- In the case of salt crystals, the atoms actually just give away an entire electron without forming a bond between the atoms. Both partners are fine with that, but they then contain a charge, because the proton-to-electron ratio is off balance. So they are now strongly attracted to each other, which leads to these super stable interactions and the regularity of a crystal lattice. They are no molecules though (of course, crystals can also be made of molecular ions)
- And then of course everything that isn’t even matter. One definition of matter is saying, it is everything that has a mass and occupies space. That doesn’t apply to photons (light particles), neutrinos (uhm… tiny cosmic stuff) and other subatomic particles. And then there are abstract things like heat, gravity, peace, veganism and grunge music.
Ultimately, most ‘things’ on our beautiful planet is made of these three structures; molecules, crystals and metals. This applies to living beings as much as to all the rest. Our cells consist of a variety of molecules, but in no way different than what we find in inanimate nature or have made synthetically. It’s all the same elements, bonds and forces.
Sometimes, when boredom finds me again (such a rare state in times of the omnipresent internet), I imagine zooming into the objects around me. Into the woolen strings of my grandma’s handmade scarf, which consist of cellulose fibrils that themselves are made from repeating glucose subunits. Or the plastic case of my phone that I am writing this post on right now. Random coils of some polymer chains that curl up in endless repetition to form a solid object. I don’t know for certain what all these sub-microscopic things look like. Not necessarily even what they are made of. But it is fun to imagine what it might be, and what the molecular structure of matter might actually look like on the tiny scale.
Musical inspiration: Gojira – Born in Winter, because the snow came back today!
For the ones who want to know more: As so often, I simplified a lot here. Atomic bonding is actually quite complex, and there are many things that I left out or didn’t properly explain. First, spin pairing: This is what happens, when two electrons occupy the same orbital, forming a covalent bond. But sometimes, they don’t actually do that and each gets their own orbital, without pairing their spins (keyword for the curios: Hund’s rule). What the electron spin exactly is, that’s a story for another day. And most importantly, the driving force for bond formation is not the spin pairing itself, because electrons repulse each other due to their charges. They are strongly attracted to the nucleus of the other atom, which is positively charged.
And then there are many cases where we form molecular orbitals (orbitals stretching over multiple atoms), that are a bit weird. For example between non-metals and metals (keyword: ligand field theory), or in the case of strong ionic bonds with actual orbital overlap. As often in science, things are not as simple as they seem at first glance, and there are a lot of exceptions to the simple cases. Nonetheless, I hope this post helped you understand the world we live in a bit better. Studying science definitely gave me a new view on the world.