The Loneliest Proton

No matter how hard you try you will never be able to grasp just how tiny, how spatially unassuming, is a proton. It is just way too small. A proton is an infinitesimal part of an atom, which is itself of course an insubstantial thing. Protons are so small that a little dib of ink like the dot on this “i” can hold something in the region of 500,000,000,000 of them, or rather more than the number of seconds it takes to make half a million years. So protons are exceedingly microscopic, to say the very least.  ~ Bill Bryson

That excerpt was blogged by Althouse without much further comment. The reader is supposed to recall from high school or college chemistry just how small the proton really is -- it is after all just a nuclear particle.

Protons cluster in every atom except for hydrogen where they appear alone.  In humans, protons mostly nucleate in groups of eight (as found in oxygen) or six (as found in carbon) with attendant neutrons, but they also go it alone in hydrogen.

Despite the proton's exceedingly tiny size in hydrogen, it is readily detected when placed in a magnetic field. They can even be spatially located in soft tissue by MRI. So there's a nice trade off. If only all the  smallest and hardest to see elements were so easy to detect.

Hydrogen is also giving us a glimpse into the mind as in MRI imaging of the brain.

"Don't call it transmutation. They'll have our heads off as alchemists"

The transmutation of elements was an ancient, discredited notion promulgated by alchemy (alchemy is to chemistry what astrology is to astronomy). Alchemists sought to turn base metals like lead into gold. They failed or were quacks and charlatans. And yet transmutation has always occurred naturally and has been practiced since 1917.

Natural transmutation was first discovered when Frederick Soddy, along with Ernest Rutherford, proved that radioactive thorium converted to radium in 1901. At the moment of realization, Soddy later recalled shouting out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."

Transmutation became a fait accompli. But it was one thing to discover that atoms could naturally and spontaneously lose little pieces like an alpha particle or a beta particle or even a gamma ray. It was quite another thing to discover that atoms could add little pieces too.

In 1917, Rutherford projected alpha particles from radium decay through air and discovered a new type of radiation which proved to be hydrogen nuclei (Rutherford named these particles protons). Further experiments showed the protons were coming from the nitrogen component of air, and he deduced that the reaction was a transmutation of nitrogen into oxygen:

¹⁴N + α → ¹⁷O + proton

This was also the first demonstrative proof of artificial transmutation and the proton's existence.  All that was needed now was the neutron which led to division and multiplication.

Flogging Phlogiston

Oxygen burned two great ideas in chemistry. Not literally, but understanding oxygen and oxidation undid two great ideas. One was called Valence Bond theory and I considered its undoing back here. VB theory is still useful and is taught in middle and high school chemistry curricula. The other great idea was called Phlogiston theory. Of course the Chinese had their own take on oxygen which you can read about here: link

The word "phlogiston" came from the ancient Greek word φλογιστόν ("burning up") and was promulgated by German scientists beginning in the 17th century. The notion had probably been around much longer because the idea was a very intuitive one. Phlogiston theory taught the existence of an element called phlogiston, a substance without color, odor, taste, or mass. Phlogiston was liberated when something burned or slowly rusted. Think of what you feel in front of an open flame. Not really so far-fetched, the notion was close to our modern notion of energy consumption, in so far as we suppose substances like fuels have "energy content." We speak of hydrocarbon's energy content in BTUs as if it were something we could distill and put in a bottle.

Phlogiston was a German notion and was undone by men like Antoine Lavoisier who showed that metals increased their mass when they burn or rust, inconsistent with something being lost or given up. Unfortunately, Lavoisier lost his head in the French Revolution for his royalist sympathies.  A dead cat bounce for Phlogiston occurred around the turn of the 19th century, just after water electrolysis was discovered.


Alessandro Volta's Pile 

When William Nicholson and Anthony Carlisle inserted the two wires from their voltaic pile together into a vessel of water, they also galvanized the entire scientific world, creating a sensation as great as any scientific discovery ever made. In Nicholson's words:

It was with no little surprise that we found the hydrogen extricated at the contact with one wire, while the oxigen [sic] fixed itself in combination with the other wire at a distance of almost two inches.

What actually happened depended on the type of metal wire used: when they used copper, hydrogen gas evolved at one wire while the other wire became "fixed with oxygen" meaning it turned to copper oxide (greenish blue). But with platinum or gold wires, hydrogen gas evolved cleanly at one wire while oxygen gas evolved cleanly at the other electrode. The great puzzle was not that the two different gases were produced, but rather that they were produced at different electrodes. It seemed to everyone that if the gases both came from the decomposition of water they should both appear at the same place.

Now the notion that hydrogen and oxygen were distinct elements was not universally accepted. It was not settled science. One of the doubters was a German named Johann Ritter. Ritter was no slouch.*  He repeated the Nicholson and Carlisle experiments and concluded that it was impossible for the gases to be produced from the decomposition of water since there was no way that a gas could travel through one wire, through the pile, and out through the other wire. The truth, Ritter argued, was that:

Water is an element

In Ritter's view, "oxygen" was just water plus positive electricity and "hydrogen" was just water plus negative electricity. He nearly set science back 2000 years. That water was an element and electricity was like phlogiston was ancient thinking. Great minds, including Michael Faraday, puzzled over water electrolysis for years. Bear in mind that in the early 1800's nobody had yet thought that water could ionize into H⁺ and OH^-. The proton (and the electron) had not yet been discovered. But the Germans ultimately lost the argument.

We now understand that water is consumed at both electrodes and electrons flow into one electrode and out the other:

At one electrode we have:  2H₂O   +   2e^-   ---->    H₂(g)   +    2OH^-
At the other electrode:        2H₂O    -   4e^-   ---->    O₂(g)   +    4H⁺  

I remember this stuff by recalling the origin of the word "oxygen" which means "acid-forming." The electrode which forms oxygen also forms acid. Of course the H⁺ and the OH^- swam the two inches back towards each other in Nicholson's experiment and remade neutral water, and completed the circuit.
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*Ritter was no slouch:

"William Herschel discovered infrared radiation because thermometers, which had recently been developed in Europe, showed a higher temperature just beyond the red end of the visible spectrum of sunlight. The German chemist Johann Wilhelm Ritter (1776-1810), after hearing about Herschel’s discovery from 1800, identified another “invisible” radiation which we now know as ultraviolet (UV) in 1801. He experimented with silver chloride since blue light was known to cause a greater reaction to it than did red light, and he found that the area just beyond the violet end of the visible spectrum showed the most intense reaction of all." reference

Umpolung: The Strange Case of Dr. Proton and Mr. Hydride

Umpolung is a chemistry term meaning "reversal of polarity." This is a useful trick, to change something from having a plus charge to having a minus charge. Hydrogen is the simplest example and shows how the same element behaves differently depending on whether it is cloaked in electrons or not.

The promiscuous proton, H⁺, flits from base to base in water. But hydride (written as H^-), laden with two electrons, usually seeks an electrophile with which to couple irreversibly. If proton and hydride get together, they make little H₂'s.

Hydrogen usually appears on the upper far left side of the Periodic Table sitting above Li, Na, K,.., emphasizing its usual cationic (H⁺) character.  But umpolung explains why it's sometimes useful to think of hydrogen as belonging on the upper right side (next to He), sitting above the halogens, F, Cl, Br,.., emphasizing its anionic H^- = hydride ~ halide character.

Acerbic Wet

The Danes and the Brits pioneered graphic depictions of simple chemical reactivity. Brønsted and Lowry independently shocked early 20th century chemists with their notions of spontaneous self-ionization of water:

Brønsted-Lowry theory explains how even the purest distilled water conducts electricity (which requires something charged). In their scheme, one water acts like a base by accepting a proton, while the other one acts like an acid, donating a proton. The slight but measurable extent of such H-swapping is real enough--a normal glass of water has a measurable concentration of H₃O⁺ of about 10^-7 units or a pH of 7 (pH is like a Richter scale). An equal & countervailing amount of hydroxide, OH^- neutralizes the acid.

Now consider adding anything to that glass of water which increases the amount of H₃O⁺ (but not OH^-).  Such a thing which donates an H⁺ to a neutral water molecule is called an acid in English.  The Germans call them Säure, which is related to our word sour. Svante Arrhenius (the august savant who also thought up AGW), came up with the idea first.