Subatomic particles from a chemist's point of view - part II: the proton
In my subjective opinion, the runner-up in this informal ranking of subatomic particles that are important in chemistry. Protons may not form chemical bonds like electrons do, but they still play an important role in many chemical reactions, especially in organic chemistry. But their most meaningful task that places them right below the electron on my list is this: they quite literally define the elements.
Let’s put our Mendeleev hats on and have a look at the periodic table. Here, I’ll upload it for you so you don’t have to google it:
It doesn’t take a genius to realize the elements are compiled in an orderly fashion rather than a random one. What is the property that generates this order? You could say mass – that the elements are arranged by their increasing mass – but that’s not quite true. Sure, most of the time it is true, but there’s a handful of oddballs that refuse to fit this scheme. Argon and potassium, for example: argon has a mass of 39,948 u (units) while potassium has a slightly lower mass of 39,098 u. The difference isn’t big, but nevertheless if we want to arrange our elements by mass, we have to place potassium underneath neon and argon underneath sodium.
Obviously, we can’t do that. The cool thing about the periodic table is that there are several trends encoded in it, one of them being that the elements of any given group are usually fairly similar to each other. Group 18, where argon normally resides, is reserved for noble gases that are extremely chill and not eager to react (they might’ve taught you in school that noble gases never ever react with anything ever; THAT’S A LIE! But it is true that their chemistry is scant and their reactions rare). Potassium could never fit in with them. Fucker explodes in water the same way sodium does – which is yet another proof it belongs in the same group! Also, COOL EXPLOSION HERE!
This isn’t the only such strange pair in the periodic table: cobalt and nickel are like that too, and so are tellurium and iodine. It isn’t much – but it’s enough that we have to look for some other physical property to define the order of the elements. For some time, chemists and physicists had to accept this discrepancy (not that they were happy about it; I imagine they’d wake up at night drenched in sweat, screaming, “GODFORSAKEN ARGON!”). The atomic number, this sort of ordinal number that put every element in its place, was actually random, as in, not based on any known physical property. Yeah, potassium has an atomic number of 19, but why?
Henry Moseley conducted a series of experiments in which he zapped various elements with X-rays (I’m so jealous), then analyzed the resulting emission spectra. It turned out that the atomic number is proportional to the square root of the emitted radiation, which in turn depends on the proton count in the nucleus. This is what defines any given element: the number of protons it has. This is THE definition, the one you learn very early in your chemistry journey. The number of neutrons may vary among the atoms of the same element (because isotopes) and atoms can gain or lose electrons by becoming ions, but that doesn’t turn them into different elements. Only the number of protons is always constant for one and the same chemical element.
Organic chemists love protons too
And for more than one reason at that – because hoo boy, does a proton stir some shit in ochem!
My ochem lab instructor pointed to the mechanism I’d written on my lab report once and asked, “What does the acid do in this reaction?”. Very plainly I said, “It’s a source of protons which act as a catalyst,” to which he gave me his standard shit-eating grin and said, “They all are.”
And he wasn’t wrong! If you analyze a bunch of organic reaction mechanisms then you’ll see they very often begin with a proton (so H+) attaching itself to the substrate (or a lone electron pair on the substrate to be precise, because Coulomb force, right?) and thus initiating a chain reaction of sorts that leads, frequently through many infuriating steps, to the product. Take a look at the synthesis of aspirin, for example:
You don’t need to understand everything that happens here. What matters is this first step I circled: a proton attaches itself to one of the substrates and starts the whole reaction.
The second reason I have in mind for why organic chemists love protons is NMR: nuclear magnetic resonance. NMR is a method of instrumental analysis and it’s cool as all fucks actually (as long as you don’t have to analyze the spectra because what the heck are those spikes), but this post is about protons, not NMR, so here’s the gist: you put your organic sample in the NMR spectrometer. The spectrometer drenches your sample in a magnetic field (which is probably why small dogs with metallic collars aren’t advised in an NMR lab). The spins of the protons in your sample (yes, protons have spin too!) go wooo! and align themselves in a specific manner. The computer connected to the spectrometer spits out a spectrum that tells you what your sample looks like.
Charge: positive one elementary electric charge, the exact opposite of an electron (how convenient!): +1.602×10^(−19) C
Mass: 1.673 × 10^(-27) kg – which is roughly 1837 times the mass of an electron. I want you to say, "Whoa, that's a lot!" right now because shit, it really is! And that's a great thing, because it gives us cool stuff like the Born-Oppenheimer approximation.
Radius: 0.841 fm (femtometers), but make no mistake: just like electrons, protons abide by the wave-particle duality, because they hate us all. I just remembered when my quantum chem professor told us during a lecture that even buckminsterfullerenes exhibit wave-particle duality. These are molecules made up of 60 carbon atoms. Sixty carbon atoms!! I almost cried, but I was sitting in the front, so I had to compose myself.
