Elemental
Page 7
In the case of the hydrogen/oxygen reaction, the hydrogen and oxygen units are floating freely, but when they react the electrons on each atom slot into molecular orbitals, linking things together and forming a bonded H2O molecule.
All the energy from these dropping electrons is released as light both visible and infrared (heat), creating the explosions first seen by Henry Cavendish.
It doesn’t have to be atoms with which we start either, it can be molecules. The bonds of a nitroglycerine molecule are at very high energy so they gladly break down into molecules with more stable orbitals, like carbon dioxide and water. A lot of energy gets kicked out in the process as all the electrons drop and we see the result as an explosion.
LET’S GET IT STARTED
The basis of chemistry is simple: start with one set of orbitals and finish with another. Prizing your original molecules apart, however, can be difficult. Electrons in a molecular orbital don’t necessarily know there’s a better deal to be had, so we have to give them a kick of energy in order to let them fall into the arrangement we want.
An analogy would be to picture a coat hanging on a coat hook. The coat will sit there until the end of time, even though it would achieve greater stability by dropping to the floor. That won’t happen because you have to put energy into the system first. It’s only when you lift the coat up a few centimeters, freeing it from the hook, that you give it the option of falling into a more stable configuration.
Electrons are exactly the same. We need to excite them first and get them out of their orbitals before they can drop into new ones.
A stable molecule like water can be thought of as having a coat hook several meters long. You’d have to get on a ladder and lift the coat all that distance to get it free. And once you let go, it would probably just fall right back onto the hook again. That’s why water reacts with hardly anything.
Nitroglycerine, on the other hand, is like a coat hook a few millimeters long, positioned over a cliff. A tiny nudge (say, from a burning fuse) is enough to get the electrons out of their orbitals, and the subsequent energy drop is enormous.
Or you could think of it like a LEGO® model. If you want to make something new, you have to put energy in and separate the blocks. It’s only when everything is broken down into its constituent parts that you can form something else.
Whatever the reaction is, chemistry is about persuading the electrons to jump out of their starting orbitals and into the ones you want. How hot do you need to get it? What shape does your starting molecule need to be? What by-products do you get? What do you do if your reaction doesn’t work? How many molecules will rearrange and how many will fall back into their original positions?
Although a myriad of complexities can arise in the lab, the overall premise is simple. Push the electrons up and let them drop down.
CHAPTER EIGHT
The Alchemist’s Dream
THE MOST EXPENSIVE ELEMENT YOU’VE NEVER HEARD OF
On April 3, 2017, the Pink Star diamond was sold at auction to Chow Tai Fook Enterprises for a cool seventy-one million dollars.1 At the time of writing this is the largest sum of money ever paid for a gemstone.
For perspective, the Hope diamond was sold in 1908 to Selim Habib on behalf of the Sultan of Turkey for $200,000, then resold in 1911 to Evalyn McLean for $154,000. In 1958, it was gifted to the Smithsonian Institution in Washington, DC, insured for one million dollars and rumored to be worth even more today.2
Diamonds are pure carbon so perhaps it would be fair to call carbon one of the most expensive elements on the table. Then again, charcoal, which is also made from pure carbon, retails for a few dollars at any supermarket. So perhaps it’s one of the cheapest.
We treat gold as a more valuable metal than silver but in the 1890s the winner of an Olympic event was presented with a silver medal rather than a gold one. Record companies reward artists with a platinum album as their highest accolade, but platinum sells for fifteen dollars less per troy ounce than gold on the open market.
Rhodium and palladium, used to make catalytic converters in cars, currently have a similar value to platinum but enjoyed a brief spike in 2008 when their value increased ten-fold, making them more valuable than gold for a month. Things are only worth as much as someone is willing to pay for them, and the elements are no different.
Plutonium is one of the most expensive materials on Earth for obvious reasons, with a value of $11,000 per gram (according to the US Department of Energy), and it’s often reported as being the most expensive element.3 But there is one other, rarely discussed, that outranks it. Californium, element number 98, is used as a starting agent in nuclear reactors and sells for a titanic twenty-seven million dollars per gram.4 The Pink Star diamond weighs about 12 g, meaning californium is over five times more expensive gram for gram.
What makes it so pricey is that californium does not occur in nature. It’s an element we have to make for ourselves.
ALL OF THEM WITCHES
Before the discovery of phosphorus and the fire experiments of the eighteenth century, chemical research was a mess. Armed with a mixture of Judaeo-Christian symbolism, ancient fairy tales, and the works of a Persian writer named Jabir ibn Hayyan, rigorous testing of chemicals was ignored and fact was mixed with superstition.
The resulting field was called alchemy, an Arabic-derived term which comes from the Greek chemia, meaning black magic. Nobody was trying to find substances that were elemental during that period. Instead, they were trying to find substances they more or less made up.
“Alkahest” was one, thought to be the ultimate acid capable of dissolving anything. The “elixir of life” was another, thought to prevent the onset of death, and the “panacea” was yet another, thought to be a medicine capable of curing all illnesses.5
Above all, though, the goal of the alchemists was to generate a material called “philosopher’s stone,” which could turn other metals into gold. Nobody knows who came up with the idea of philosopher’s stone but rumors of its existence had been circulating since the thirteenth century.
The author of a medieval encyclopedia, Vincent of Beauvais, claimed that God had imparted knowledge of “transmutation” to Adam, who passed it on to Noah and so forth. His source for this seems to have been his own imagination although The Book of Sydrac, an anonymous thirteenth-century text, tells a similar story, so it was obviously a common idea at the time.6
One of the earliest recorded references to the phrase “philosopher’s stone” is in a 1610 play called The Alchemist by Ben Jonson, which suggests that Adam was told how to make the fabled substance.7 After he got kicked out of Eden, presumably he forgot the recipe. Nice going, Adam! First you lose a rib, then you lose the philosopher’s stone recipe. What’s next? Your second-born son?
Alchemy did give us knowledge about various chemical reactions, not to mention Brandt’s discovery of phosphorus, but it had no structure to it and there was more guesswork than anything else.
The problem with trying to turn one element into another is that an element’s identity is determined by the number of protons in its nucleus and changing that isn’t a simple matter of mixing things in a test tube.
As we saw in the previous chapter, chemistry is all about manipulating electrons. The nucleus is too small and hidden for us to have any impact on it. Simply put, electrons can dance to any tune you want but if the nucleus remains untouched the element remains the same.
And yet suns are constantly transmuting hydrogen into helium so there is obviously no law of science that forbids it from happening. To mimic the technique here on Earth would take superhuman powers. Speaking of which …
THE ORIGIN OF SUPERHEROES
Peter Parker got his Spider-Man powers when he was bitten by a radioactive spider and his DNA became irreparably altered. Bruce Banner got caught in the blast of an atom bomb and was belted by radioactive gamma rays, turning him into the Hulk. The Fantastic Four were caught in a storm of radioactive cosmic rays, Daredevil
was splattered with radioactive waste, and Jean Grey of the X-Men (in the original storyline) released her telekinetic potential from flying a shuttle through a radioactive solar storm.8
Radioactivity has obviously given us much to be thankful for, but it also created Godzilla and an uncountable number of giant insects during the 1950s so we should probably treat it with caution.9 Nevertheless, it was through radioactivity that humankind was finally able to transmute one element into another, so we need to get acquainted with it.
The phenomenon was discovered by accident in 1896 by the French physicist Henri Becquerel. Becquerel had been planning to do some experiments with photographic plates but on the day of his tests the sky was overcast, so he put them in his drawer.
Two days later when he got them back out, the plates had somehow been impregnated with the image of a copper cross lying next to them. Apparently, something in the drawer had taken a photograph. The only other object present was a jar of potassium uranyl sulfate solution on the other side of the cross so Becquerel decided it had to be the culprit.
While a photographic plate is best activated by sunlight, any high-energy beam will cause it to undergo a change. Potassium and sulfate particles don’t emit beams, so logically it was coming from the other element in the liquid, uranium.
Invisible to the human eye, the uranium was apparently emitting something that altered the surface of the photographic plates. The cross had got in between them and, voilà, the first radiogram taken by a jar.
Soon after Becquerel’s discovery, Marie Curie, the only person to win Nobel Prizes in two sciences, named the phenomenon radioactivity from the Greek radius (wheel spoke) and the Latin aktinos (ray).
With her husband Pierre, Marie discovered two more radioactive elements, which she named radium (for obvious reasons) and polonium after her home country of Poland. Sadly, both the Curies succumbed to illnesses caused by their exposure to radioactivity, which taught us something else—it’s damaging to cells.
INHERENT INSTABILITY
As we learned in Chapter 3, the nucleus of an atom is an unstable design. While the protons hold electrons in place, they also repel each other, which requires neutrons to glue them together.
The Austrian-born scientist Lise Meitner figured out that once you got up to elements around the high eighties, this equilibrium would become unstable and the nucleus could fall apart. For this important discovery, Meitner did not win the Nobel Prize for physics. Her male lab partner did. But Element 109 was eventually named meitnerium after her, so she hasn’t been snubbed completely.
As we ascend through the elements, proton numbers increase so the neutron numbers have to follow suit to keep things together. But there’s a complication (isn’t there always?). The repulsive force between protons has an infinite range but the glue force from the neutrons doesn’t.
This means that in large atoms it’s only a matter of time before repulsion wins, making them precarious structures. Larger atoms are fragile and left for long enough will break apart.
Blue-glowing actinium has a colossal nucleus of eighty-nine protons so, if you have a lump of it, around half will decay into something else within twenty years. Rubidium by contrast is much smaller, with only thirty-seven protons, and takes forty-nine billion years to decay by the same amount.
The nuclei these elements turn into tend to have peculiar numbers of neutrons, which the element doesn’t normally have. These “daughter” particles can only be produced from radioactive decay, so if you measure the amount of mother and daughter nuclei in a rock, the ratio between them allows you to work out how much you had to start with and subsequently how long it’s been around for.
It was using this technique that the American chemist Clair Patterson calculated the age of the Earth to be approximately 4.5 billion years old.10
BREAK IT DOWN
There are different ways a nucleus can decay. Sometimes the whole thing will split in what we call fission, but for reasons that aren’t understood the most common thing to get ejected from a nucleus when it fractures is a bundle of two protons and two neutrons moving at tremendous speed.
These packets come hurtling out of their atoms at 15 million meters per second, and turn out to be the very same alpha particles Rutherford used in the gold-foil experiments.
When an alpha particle is emitted the nucleus left behind has lost two protons, changing its identity. Rutherford decided to use this to his advantage. Given the velocity of alpha particles, he proposed that if you shot them at another atom they could shatter its nucleus, turning it into something lighter.
By firing alpha particles through a highly pressurized container of nitrogen gas to increase the chances of collision, Rutherford was eventually able to bash nitrogen atoms apart, turning them into carbon, in 1919. His experiment made headlines because he had “split the atom” and achieved transmutation between elements. The long sought-after dream of the alchemist was not a mythical stone from the Garden of Eden: it was a gas chamber and an alpha-emitter.11
Turning lead into gold might not be possible through sacred incantation but, if you take an element like thallium, boil it to a gas, pressurize it, and fire alpha particles through the sample, one in every few thousand thallium atoms would be turned into gold.
BUILD IT UP
Alpha decay makes a certain amount of sense to the human mind because we can imagine something falling apart when repulsion overcomes attraction. There’s another thing that can happen inside the nucleus, though, which can’t be visualized so easily. Neutrons can turn into protons and spit out an electron as they do so.
There’s a detailed account of how this happens in Appendix IV but it would take us way off track at this point. The best thing to do is cheat and think of a neutron as being a proton with an electron wrapped around it like a candy wrapper. If the electron is peeled off and discarded, the resulting particle will be a proton.
We call these streams of ejected electrons beta radiation and, unlike alpha decay, which only happens to heavy nuclei, the neutron/proton transformation can occur in any element. Some are more susceptible than others (those with more neutrons) but any atom is potentially beta radioactive.
If we could persuade an element to turn one of its neutrons into a proton, we could obtain an element one number higher than that with which we started: Rutherford’s process in reverse. But first, bananas.
BANANAS
Radioactive particles are charged and move at high speed, which means they destroy things in their path including the chemicals of your body.
If you become exposed to enough radioactive beams the DNA in your cells will fall apart and your body will disintegrate from the inside out. Usually, the fastest-growing parts (hair and nails) get affected first, which is why radiation sickness causes them to fall out. Then all sorts of lovely things happen like your skin peeling off, your teeth dropping out, and your innards gradually dissolving into a disordered mush.
In order to monitor the radioactivity to which a person is exposed, we measure the dosage in units called sieverts. A sievert is how much energy a radioactive beam is carrying, compared to the mass of the person it’s entering.
There aren’t clear figures on how many sieverts are dangerous to a human but roughly five-hundredths of a sievert per year is when things become problematic.12 The most radioactive thing you’re ever likely to come across is a dental X-ray or a mammogram scan, which delivers approximately 0.0004-hundredths of a sievert. A completely safe dose, in other words.
There is another unit that can be used to measure radioactive exposure: the banana.
The first thing to say here is that certain nuclei are more stable than others. By using the Schrödinger equation for protons and neutrons, we can obtain a list of especially stable nucleus values, which are genuinely called “magic numbers.” There isn’t an agreement on why this works—we just know that certain numbers of protons and neutrons are good and others are bad.
Potassium is a prime example. Mo
st of the potassium atoms in the Universe are stable with nineteen protons and twenty neutrons, but around 0.012 percent have twenty-one neutrons instead, making them potassium-40, and this configuration happens to be unstable.
Potassium-40 will undergo beta decay readily so any sample of potassium will be emitting a very faint trickle of radioactivity, and the fruit that contains the most potassium is the humble banana.
Originally created as a joke in 1995 by Gary Mansfield at the Lawrence Livermore National Laboratory, the Banana Equivalent Dose (BED for short) calculates the amount of radioactivity one is likely to experience from eating a single banana and can be used to calculate the radioactivity of your food.13
Don’t be alarmed, though: one BED comes to just under one-millionth of a sievert, so before you boycott bananas let’s run the math. If we assume five-hundredths of a sievert per year is lethal, you’d have to consume five thousand bananas fast enough for it to be dangerous. That’s fourteen bananas a day. For a year.
If you really want to attempt this experiment, then I suggest you consult a doctor first. And probably a psychiatrist.
BACK TO PLAYING GOD
In 1940, the American chemist Dale Corson isolated element 85, astatine.14 Predicted by Mendeleev’s table, it was the last natural element to be discovered.
Slotting it into place gave us a periodic table that went all the way from 1 to 92 without any gaps. From hydrogen formed in the big bang to uranium formed in supernovae, every element was finally identified. But could we go further and generate our own with higher numbers?