By Ron B. Davis Jr., Georgetown University
About 14 billion years ago, all of space, time, matter, and energy was released into our universe in an event known as the big bang. In the nanoseconds following this event, the universe was mostly energy. But as expansion quickly took place, a transformation happened. Some of that energy converted into matter, and the creation of elements began.
Fusion of Atoms
Not all 118 atoms of the table were created in the inconceivably massive energetic event, the big bang. In fact, only three were made—hydrogen, helium, and lithium. Every other element on the table would have to be created using just these three ingredients.
To make a larger atomic nucleus from a smaller one, protons must somehow be brought together to increase the atomic number of the resulting nucleus. We call this process of bringing together the nuclei of two atoms ‘nucleosynthesis’ or ‘nuclear fusion’.
But fusing two atoms together by combining their positively-charged nuclei isn’t easy. Physics predicts that two positively-charged particles will repel one another very strongly. This energetic barrier that must be overcome to get two atomic nuclei to fuse into one is called the Coulomb barrier.
To overcome this barrier requires that atoms be mashed together under tremendous pressure and temperature. These sorts of conditions exist in nature, but only in the massive cosmic furnaces that we call stars.
Nuclear Binding Energy
However, there is one more troubling thermodynamic riddle that seems to predict larger atoms shouldn’t be stable, or might not exist at all—the concept of entropy.
Simply put, entropy is randomness, and many years of detailed work in physics have given us the second law of thermodynamics, which predicts that the universe trends toward systems of greater disorder. That means, lots of small atoms, rather than smaller numbers of large atoms.
Fortunately for us, there is a force that counterbalances this tendency of matter to want to be many smaller particles, rather than a few large ones. It is a somewhat mysterious force known as nuclear binding energy.
Nuclear binding energy is a measure of how much stability is added to a nucleus by its nucleons. Each atom’s protons exert electrostatic repulsion that pushes particles apart. But that is balanced against some very complex interactions among the nucleons that helps an atom stay together. That’s nuclear binding energy.
The Thermodynamic Perspective
If we plot the nuclear binding energy per nucleon of common atoms as a function of their atomic mass, we observe an interesting trend. We see that lighter elemental nuclei, like those of hydrogen, helium, and lithium, are actually less stable than many elements on the second, third, and even fourth rows of the table.
In fact, the sweet spot on this curve, where nuclear binding energy is at its greatest, is the twenty-sixth element, iron; specifically, the iron-56 isotope.
What this means is that from a thermodynamic perspective, small atoms are on a quest to become something larger. Every hydrogen, helium, and lithium atom in the universe aspires to be something larger. And elements get their chance in the bellies of stars.
The Oddo-Harkins Rule
Our sun is about 109 times the diameter of Earth, and about 333,000 times more massive! The temperature near its core is thought to reach 27 million degrees Fahrenheit and pressure near the core is 10,000 times greater than at the center of the Earth. It is a huge, high-pressure, furnace that is constantly fusing hydrogen nuclei into helium nuclei, releasing tremendous energy in the process.
But under those conditions, some of that helium can undergo additional fusion, forming row two elements like carbon, nitrogen, and oxygen—elements 6,7, and 8. Helium’s role in this process is evident in what is commonly called the Oddo-Harkins rule, which states that even atomic-numbered elements are more abundant in the universe than odd atomic-numbered elements of similar size.
One day, the Sun will run so low on hydrogen fuel that it will collapse in on itself, and in doing so create a high-pressure system that is expected to produce atoms as large as element-14, silicon.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Effects of Supernova Explosions
Other cosmic events that create even greater pressure and temperature than a red giant star—such as the death of high mass stars—can produce unfathomable conditions of pressure and temperature: conditions under which even iron and nickel atoms are forced to fuse with other nuclei yet again to produce elements as massive as plutonium.
Supernova explosions and stellar collisions finally spread these nuclei into the universe, where they can seed clouds of matter that are just forming new solar systems.
It is widely believed that this is exactly what happened in our solar system—that the disk of loose matter that would ultimately form our sun and planets was seeded by the remnants of one or more supermassive stars that created a full set of elements as they perished 5 billion years ago.
Primordial Elements
Elements as large as plutonium can be produced in supernovae, but the plutonium produced in this process is radioactive. Its most stable known isotope decays with a half-life of about 80 million years. So, any plutonium that may form would be long gone within 10 times 80 million half-lives, or 800 million years.
But uranium, element 92, has an isotope with a half-life of about 4.5 billion years. With that sort of longevity, uranium atoms can form, be ejected and cross the galaxy, and find a home in a solar system like ours long before all those nuclei decay. The same is true for the element thorium, whose most stable isotope has a half-life nearly the age of the universe.
We can call elements like uranium and thorium ‘primordial’ elements since the atoms making up these elements on Earth have mostly existed since before the creation of our planet.
Common Questions about Nucleosynthesis
The energetic barrier that must be overcome to get two atomic nuclei to fuse into one is called the Coulomb barrier.
Nuclear binding energy is a measure of how much stability is added to a nucleus by its nucleons. Each atom’s protons exert electrostatic repulsion that pushes particles apart. But that is balanced against some very complex interactions among the nucleons that helps an atom stay together. That’s nuclear binding energy.
Elements like uranium and thorium are called primordial elements since the atoms making up these elements on Earth have mostly existed since before the creation of our planet.
