By Ron B. Davis Jr., Georgetown University
The periodic table helps us to understand the elements around us and how they have become distributed in our environment. To better understand the distribution of elements and where we find them in our immediate cosmological environment, it’s crucial to focus on the three elements forged in the Big Bang and the other elements forged in the cores of stars.
Elements on Earth
Across the universe, hydrogen is by far the most common element, accounting for about 74% of all mass. That includes stars, galaxies, and even atoms in intergalactic space. Helium makes up nearly all of the remainder, at about 24%.
But if we take a look around us, there is aluminum in our kitchen silverware; iron, cobalt, nickel, and carbon in the steel that makes up parts of our car; sodium and chlorine dissolved in the world’s oceans; and the oxygen needed to make all the H2O in our oceans. There certainly do seem to be a lot of other elements than hydrogen, helium, and lithium. And they seem to be much more abundant on Earth than their average appearance in the universe as a whole.
Supernovae Forged and Distributed the Elements
The exact process by which our solar system was created is unknown. One thing that we do know, however, is that it must have formed from a cloud of interstellar gas called the solar nebula that originated with material from supernovae and other events involving the death of very large stars.
We know this because cataclysmic events involving very large stars are the only reasonable way to explain the fact that heavy primordial elements like gold, lead, and even uranium—all much larger atoms than iron—make up a part of our solar system.
The very presence of these elements proves that everything around us contains atoms that were forged in a super-massive star and ejected in an explosion so powerful that it released more energy in an instant than our sun will release during its entire 10-billion-year lifespan.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Distribution of Elements on Earth
Everything around us is literally made of stardust—atoms formed in burning stars and supernovas. That helps to explain the menu of about 90 elements that nature had to work with when forming our planet. But it doesn’t explain the remarkable variety of compositions that we see around the Earth.
Earth’s atmosphere is made principally of nitrogen, oxygen, argon, and a few molecular compounds like water and carbon dioxide. By contrast, the inner core of the planet is composed almost completely of iron. But why is that? And how might the periodic table help us understand which elements are found where?
The Distribution of Iron
Let’s refer to the creation of Earth. In the early days of our planet’s existence—the first 500 million years or so is a geologic period that geologists call the Hadean—our newly-formed planet had just come together.
It was a hot, churning ball of elemental material—no longer a nebulous gas cloud, but an accreting ball of hot molten material, heated by the energy released as atoms came together under the influence of gravity. The molten state of this early Earth made it inhospitable to life. But these conditions gave the elements freedom to move and accumulate anywhere they pleased.
Iron, silicon, and oxygen are the three most abundant on our planet today, so let’s take a moment and consider how these three elements moved through the early Earth as it slowly cooled.
From row 4, iron’s most common isotope has a mass of 56. This makes its nucleus twice the mass of silicon and more than three times the mass of oxygen. This makes iron extremely dense by comparison, so in a slowly cooling, molten earth, iron would naturally accumulate deep within the core as it sinks through the lighter material around it. Thus, Earth’s core consists overwhelmingly of iron.
The Distribution of Silicon, Sulfur, and Oxygen
In the meantime, silicon and oxygen suffered a very different fate. The Hadean Earth’s atmosphere did not contain much oxygen at all. In fact, most of the oxygen in our fledgling planet was instead busy bonding with the element silicon to form silicon dioxide. Made of much less massive elements, its pure form, SiO2, or quartz, has a density of about 2.7 (less than half that of iron).
So as the dense metallic iron sank to the core of the molten planet, the more buoyant silicon dioxide instead moved upward, finding a home much closer to the surface. A great deal of sulfur was also present, and as one might imagine from its position on the table, much of that sulfur made its way to the surface along with the silicon and oxygen from the same region of the table.
In the field of chemistry, there is an old adage, “like dissolves like”. We can think of these most common materials—whether iron, silica, or sulfur—as solvents for other elements during the Hadean Earth. So, as these solvents sank or rose, they drove friendly elements downward or upward with them as the planet differentiated.
Common Questions about the Distribution of Elements around Us
Earth’s atmosphere is made principally of nitrogen, oxygen, argon, and a few molecular compounds like water and carbon dioxide.
During the formation of the Earth, when it was a hot molten sphere, elements had the chance to move freely and accumulate where they wanted. So the distribution of elements occurred during that time. Dense elements, such as iron, made their way to the inner core.
During the Hadean Earth, less dense elements such as silicon and sulfur made their way upward near the surface of the Earth. The oxygen was busy bonding with silicon to make silicon dioxide. So this element accumulated on the surface along with the silicon.
