How to Make an Element

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff. --Carl Sagan

Almost all of the elements in the universe originated in the high-pressure hearts of stars or during a star's violent death. But some elements are not "star stuff." Hydrogen and helium trace their lineage back to the big bang. Other elements, like francium and plutonium, are only produced in trace amounts by the decay of uranium--and by trace amounts, I mean that if you gathered all the naturally occurring plutonium in the world, you'd have roughly 0.05 grams of it.

In fact, the periodic table might well have ended after plutonium if scientists had not picked up where nature left off. In the last 75 years, scientists have added an additional 24 elements to the periodic table and created several others that are so rare we can only speculate about their existence in nature. We have redefined matter and are perhaps the only planet that has ever seen elements heavier than plutonium. But how much further can we go?

To answer this question, it is important to understand the anatomy of an element. Everyone has heard of the periodic table of elements, or at least seen it hanging on the wall of a high school science classroom. This infamous table classifies all the atoms in the universe into 118 different types, known as elements. An atom is composed of two parts: a nucleus and an electron cloud. The nucleus is in the heart of an atom and composed of positive particles called protons and neutral particles called neutrons. Negatively charged electrons buzz around the nucleus in the electron cloud.

Lithium atom
A stylized view of the lithium atom. Made by Halfdan. Via the Wikimedia Commons.

An atom looks a little like the picture to the right--except atoms are a thousand-million times smaller than you see them here. (If this were the real size of an atom, you would be roughly the size of the sun!) Even though all the components of an atom are important, the periodic table ignores the number of neutrons and electrons and defines an atom based solely on the number of protons it has in its nucleus. All atoms with six protons are carbon, regardless of how many neutrons or electrons they have; nitrogen is element seven because it has seven protons, and so on until we reach ununoctium with 118 protons.

So creating a brand new element requires loading an atom's nucleus with more protons. Stars create new elements in their cores by squeezing elements together in a process called nuclear fusion. First, stars fuse hydrogen atoms into helium. Helium atoms then fuse to create beryllium, and so on, until fusion in the star's core has created every element up to iron. Iron is the last element stars create in their cores, and a kiss of death for any star with the moxie (that is, the mass) to make it to this point. As astronomer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics describes it, "Once a star has built an iron core, there is no way it can generate energy by fusion. The star, radiating energy at a prodigious rate, becomes like a teenager with a credit card. Using resources much faster than can be replenished, it is perched on the edge of disaster."

But the edge of disaster for these massive stars is the threshold of life for the rest of the periodic table. In a star's last second of life, its core compacts so tightly that it becomes as dense as an atomic nucleus. When no more matter can squeeze into the core, the star explodes with the energy of an octillion (10^27) atomic bombs. In this violent explosion, more than half the elements on the periodic table are born. Intense heat from the explosion catalyzes nuclear reactions that were not possible in the core. Escaping elements are bombarded with neutrons, which split inside the nucleus into protons and electrons, generating new unique elements. Iron turns into gold, gold turns into lead, and so on until uranium, the heaviest naturally star-born element, is forged from the ashes.

This spectacular shower of life and death creates everything. Well, almost everything. There are another 27 elements on the periodic table after uranium that were not created by stars. Some elements are produced in trace amounts by the decay of other elements. But even the long radioactive decay chain is not enough to produce the ultra-heavy elements at the end of the periodic table. The periodic table would have ended altogether if scientists had not pushed the boundaries of natural physics and ventured deeper into the world of super heavy elements.

To make new elements, scientists borrowed some advice from the heavens. The transuranium elements (elements 95 through 100) were forged by bombarding uranium with neutrons and waiting for the impregnated nucleus to become radioactive and convert its extra neutron into a proton, electron, and a charge-less, nearly massless, antineutrino. But after fermium (element 100), the irradiate-and-wait technique stops working. Particle physicists "stepped up their game" and upgraded their atomic fodder from neutrons to other elements. The trick was to get the nuclei of the two atoms to fuse into one giant nucleus, generating an entirely unique atom. Scientists started small--firing helium (2) at einsteinium (99) to beget mendelevium (101); launching neon (10) at uranium (92) to engender nobelium (102). Eventually, scientists busted out the big guns and bombarded lead (82) with zinc (30) to beget copernicium (112) and californium (98) with calcium (20) to produce element 118, provisionally called ununoctium.

But why do scientists succeed where the stars fail? The truth is, the stars don't fail. In the storm of their deaths, some stars probably do forge super heavy elements--even elements heavier than we've created--but these elements don't survive long in the turbulent chaos of a supernova. Super-heavy elements are so fragile they live only a matter of microseconds before they decay into a jumble of atomic scrap metal.

There is a limit to the number of protons and neutrons that can squeeze inside an atomic nucleus, but we haven't found it yet. Protons are positively charged, and because like-charges repel, the protons are in a continuous "this nucleus ain't big enough for the both of us" duel. The neutrons have no charge and quell some of the tension by weaseling between the protons. The entire nucleus is held together by the strong force--a mysterious force that acts like a bungee cord and pulls everything together. But eventually, the proton's repulsion overwhelms the strong force, and not even the neutral neutrons can prevent the emigration of alpha particles (two neutrons and two protons) from the nucleus. So the real question is: How big can we go?

As we close the gap between what does exist and what can exist, the laws of physics will eventually stop us from venturing deeper into the world of synthetic matter. Scientists will continue to push the limit of "physically possible," but for now it appears the periodic table is nearing its completion.


Who Let the Cat out of the Bag?

"Erwin Schrödinger is going through airport security when an official asks to check his bag. After opening the bag, the official is appalled and shouts, "Sir, did you know there is a dead cat in your bag!" And Schrödinger calmly replies, "Well now there is."

If you're asking, "Who is Schrödinger? And why does he have a dead cat in his bag?" you probably missed the punch line of the joke. Don't worry--we'll get there, but before we investigate Schrödinger's "cat in a box" quantum-blurring-mind-boggling thought experiment, I want you to take a deep breath and plug your nose--because we're about to dive into some deep quantum mechanics.

Photograph of Toby the cat copyright Kevin Steele.

Quantum mechanics is a branch of physics that scientists use to describe the behaviors of small particles (like electrons). But unlike classical physics--which describes the behaviors of big objects like baseballs and rockets--quantum mechanics doesn't deal in nice exact answers. Instead, it deals in probabilities. For example, if I asked the question, "Where is Suzy?" classical mechanics would predict, "Suzy is on the couch," while quantum mechanics would tell us that "Suzy is probably on the couch, but she might also be in the bathroom, or walking in the garden, and there is a small but nonzero chance that Suzy is currently enjoying tea on the far side of the moon."

So if scientists can use classical mechanics to predict the exact trajectory of a NASA spacecraft headed for Mars, why can't they use quantum mechanics to predict something as simple as the location of an electron?

Warning: This is where things start to get weird...and a little disturbing. There is an inherent indeterminacy embedded in quantum mechanics that prevents scientists from predicting variables like position and momentum exactly. But what causes this indeterminacy? Ask Einstein and he would say indeterminacy is a reflection of our own ignorance. But ask Niels Bohr and he would argue that particles don't have finite positions or momentums until they are measured by an observer, at which point the act of measurement itself forces the particle to "take a stand" and choose a state. Pascual Jordan, one of the fathers of quantum mechanics, put it this way: "Observations not only disturb what is to be measured, they produce it...we compel (the particle) to assume a definite position."

This is the underlying concept of the "cat in the bag" joke. Erwin Schrödinger, one of the masterminds of quantum theory, devised a thought experiment purely to highlight the absurdity of Bohr's interpretation. In his thought experiment, Schrödinger sets up a scene where a cat is placed in a box with one of Bohr's "indecisive particles," but the life or death of that cat depends upon which state the particle chooses. If the particle chooses one state (let's call it state A), the cat lives, but if the particle chooses the other state (state B), poisonous gas is released into the box and the cat dies. Applying Bohr's view of indeterminacy to this situation, the particle doesn't have a definite state and exists as a sort of hybrid--both A and B at once--that physicists call a "superposition." But think about what this means for the cat. If the particle exists in a superposition of states A and B, the cat also must exist in a superposition of two states, dead and alive. But as soon as an observer opens the box (or bag), the sheer act of observation "compels" the particle to exist in either A or B, and thus the cat must be either dead or alive. This situation is rather awkward if the first observer is oblivious to the experiment--like airport security.

Why would Bohr advocate something that sounds so ridiculous? And why would Schrödinger's derisive analogy become the poster child of quantum theory? Even today there is no consensus on a "right" interpretation of quantum theory. (For more on this debate, visit NOVA's physics blog, The Nature of Reality.) However, experimental results confirm that measurements (like peeking inside Schrödinger's bag) can affect and even determine the state of quantum systems. Quantum particles behave like they "know" when they're being watched, and adjust their behavior accordingly, like a group of mischievous youngsters keenly aware of an adult presence in the room.

Perhaps the "measurement problem," as it is called, is not all that strange--it merely seems so because we lack the words to explain it. If a quantum particle tried to convey to us what it feels like to be a quantum particle, it would be like a cube explaining to a square what it feels like to be 3-D. The only language that bridges the two different worlds--quantum and classical, 2-D and 3-D--is mathematics. And mathematically, we can describe this strange quantum world incredibly accurately using probability.

Happily for cats, today Schrödinger's thought experiment is used only as a mascot for quantum theory and not as a standard of thought. Theorists are still working to explain the measurement problem with fresh interpretations of quantum mechanics that could resolve the apparent paradox. So cats everywhere are least for now.


Finding Life Beyond...Carbon?

If you watched Finding Life Beyond Earth last week, you might be wondering, what's the big deal about carbon-based life? Spock discovered silicon-based life in episode twenty-five of the original Star Trek series. And Star Wars' bounty hunter Zuckuss is ammonium based. How can scientists say these two hegemons of geek culture are wrong? Why does life have to be based on organic molecules?

The answer to this question is actually right in front of you. I'll give you a hint--it's big, colorful, and located directly below this paragraph. It's the periodic table! The information in the periodic table of elements is sufficient to convince scientists that life has to be carbon based--you just have to know how to interpret it.

The answer is in here! Image via Wikipedia

Let's start with carbon's box on the periodic table. Carbon is element number six, which tells us that it has six negatively charged electrons buzzing around six positively charged protons in the nucleus. That's the information we're given--now let's apply a basic knowledge of chemistry and interpret it.

Atoms house their electrons in concentric energy shells. The first energy shell holds two electrons, the second and third hold eight. (There are more, but let's stop here for now.) Atoms fill their shells sequentially, and they always like to end with a full shell--even if that means giving away electrons. Sodium, for instance, has one electron dangling in its third energy shell and instead of trying to gain seven more electrons, sodium simply gives this electron away. Other atoms steal electrons to fill their outermost shell. Chlorine is one electron short of filling its outer shell, so it plucks an electron from an element, like sodium, and turns into a stable, negatively-charged version of itself.


Heisenberg Humor

Professor Werner Heisenberg is speeding down the highway, when a cop pulls him over. The cop walks up to his car and asks, "Excuse me sir, do you know how fast you were going?" And Heisenberg responds, "No...but I know exactly where I am!"

Watch the full episode. See more NOVA.

If you understood this joke, read no further. However, if you're still a little confused, get ready to dive into some uncensored quantum mechanics. (Just kidding, I've removed the nastiest bits for you!)

This joke is based on the Heisenberg uncertainty principle--one of the fundamental tenets of quantum mechanics. The uncertainty principle can be summed up as: "The more precisely the position of a particle is determined, the less precisely the momentum is known in this instant, and vice versa." But to understand this statement, and hence the joke, it is first necessary to take on some quantum mechanics.

Quantum mechanics describes the behaviors of small particles, like electrons whizzing around an atomic nucleus. Compared to classical mechanics, which explains the behaviors of big objects like baseballs, airplanes, skiers--basically anything you encounter in your daily life--quantum mechanics seems very strange. To understand quantum mechanics, you need to let go of everything you've ever observed on the macro-scale and accept that there is a tiny world governed by its own unique laws.

The first principle of quantum mechanics you need to know in order to understand the joke is this: Subatomic particles are inherently fuzzy. An electron is not a fireball zooming around a chunky nucleus, but a negatively charged blanket that sometimes acts like a sheet crumbled into a ball and sometimes like a quilt spread across a bed. And because of this inherent fuzziness, the properties that describe the behaviors of these particles--such as position and momentum--are inherently fuzzy as well.

But the uncertainty principle is more specific than "particles are fuzzy." It holds that it is possible to determine either the position or the momentum of a particle, but it is impossible to determine both simultaneously. In other words, the more precisely we measure the momentum of a particle, the less precisely we can measure its position, and visa versa.

Are you laughing yet? Or are you beginning to wonder why the act of measurement seems to have such elevated importance? The way Heisenberg saw it, measuring any observable property of a particle actually affects that property. In this view, an electron doesn't even have a finite position and momentum until a scientist attempts to measure it, at which point the electron is forced to choose a state--like an atomic game of musical chairs where each player exists in every chair until the music stops. Not every physicist likes this interpretation of quantum mechanics--it always galled Einstein--but it's the one I'm sticking to for simplicity. After all, we're just trying to understand a joke here!

Still no guffaws? Okay, then it's time for an equation. As perplexing as the uncertainty principle is, it is represented by a very simple one: The product of the uncertainty in position and the uncertainty in momentum will always be greater than or equal to a constant. In other words, these two uncertainties are inversely related--if one increases, the other falls by a proportional amount.

However, the uncertainty principle can only be directly observed on the atomic scale. So sorry, Dr. Heisenberg, but it looks like you're getting a ticket after all.

Sarah Charley

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