By Ron B. Davis Jr., Georgetown University
Silicon finds itself in a difficult place when choosing just how to combine with oxygen. It doesn’t form ionic compounds like metals, and it lacks the ability to make discrete covalent molecules like many nonmetals. So instead, silicon creates a vast network of covalent silicon oxygen bonds to form the mineral known as silica.
Forms of Silica
When pure, silica can appear in two forms, either as glass, in which the silicon and oxygen atoms are still bonded, but are not arranged in any particular repeating pattern, or as quartz, in which the atoms are in a highly ordered, crystalline pattern.
At about 1,700° Celsius, heat finally wins the battle with the silicon-oxygen bonds in silica, melting it into a flowing viscous liquid.
Silica makes up that base for the silicate minerals that slowly rose to the surface of the Earth in its infancy, carrying with them lithophilic elements like sodium, potassium, aluminum and even uranium as they rose. So, in many ways, we owe the composition of the Earth’s crust to the unusual chemical behavior of this very special metalloid.
Like most small elements with even atomic numbers, silicon is around us in great supply. It is second only to oxygen as the most common element in Earth’s crust. Also, silicon participates in the formation of a huge host of rock-forming minerals.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Scarcity of Boron
Even boron, at element number 5 on the periodic table, comes nowhere close to the abundance of silicon. Instead, boron’s natural abundance in both our immediate environment and the solar system is fairly low.
The reasons for the relative scarcity of boron start all the way back to the very beginning, where element 5 was just a bit too large to have formed during the big bang. And then, element 5 is an odd-numbered nucleus that is often passed over during the stellar nucleosynthesis process. A quick check of the nuclear binding energy curve also confirms that boron, much like beryllium, has a nuclear binding energy that is actually lower than that of helium, making its formation in stars energetically disfavored.
All of this adds up to boron being relatively rare throughout the universe, despite having such a small nucleus. And although strikingly rare for such a small element, boron’s abundance in our environment is still on par with many larger, very familiar elements like copper, nickel and zinc.
Tellurium’s Rarity
Moving on to tellurium, its atoms are quite massive being from the fifth row of the table. We certainly would expect it to be less abundant than metalloids from higher rows. But tellurium has an especially low abundance that was a real puzzle for scientists for many decades. Tellurium is an even-numbered element whose cosmic abundance is known to be about ten times greater than antimony’s, as would be suggested by the Oddo-Harkins Rule.
But here in the crust of the Earth, tellurium’s rarity is rivaled only by that of precious metals like gold, platinum and palladium. And yet, tellurium is a surface-tending chalcophile, not a siderophile that sinks to Earth’s core. So, how did this happen?
The answer to tellurium’s remarkable scarcity on the surface of the Earth goes far beyond having a row-5 nucleus. Instead, it is due even more to its position in oxygen-sulfur column, the chalcogens.
Tellurium on Earth
Tellurium, just like its smaller nonmetal siblings in group 16, tends to form small molecular compounds in combination with hydrogen. In the presence of hydrogen, oxygen easily forms H2O, sulfur forms H2S, and tellurium can form H2Te.
By combining so effectively with such small atoms, most of the tellurium in or near the newborn planet Earth had one of two fates. First, much of the tellurium is thought to have reacted with more abundant hydrogen in the early atmosphere, making hydrogen telluride gas, which has a boiling point of just –2° Celsius. On the hot, early Earth, this volatile, gaseous material could very well have escaped easily into space, leaving precious little tellurium in the crust for humans to discover 4.5 billion years later.
But, tellurium is also a chalcophile, which means it has a ‘sulfur-loving’ character that would have allowed a small amount of it to find safe harbor in solid, sulfur-rich minerals that made their way to the surface and stayed put. In some of those minerals, tellurium impersonates sulfur, taking its place in the mineral, while in others, it acts more like a metal, bonding with sulfur to form a solid.
This two-sidedness is a recurring theme for the metalloids. In the scenario we just discussed, tellurium had the ability to behave both like a metal and a nonmetal.
Common Questions about the availability of Metalloids Silicon, Boron and Tellurium
When pure, silica can appear in two forms, either as glass, in which the silicon and oxygen atoms are still bonded, but are not arranged in any particular repeating pattern, or as quartz, in which the atoms are in a highly ordered, crystalline pattern.
Boron is element number 5 on the periodic table. The reasons for the relative scarcity of boron start all the way back to the very beginning, where element 5 was just a bit too large to have formed during the big bang. And then, element 5 is an odd-numbered nucleus that is often passed over during the stellar nucleosynthesis process.
Tellurium is a chalcophile, which means it has a ‘sulfur-loving’ character that would have allowed a small amount of it to find safe harbor in solid, sulfur-rich minerals that made their way to the surface of planet Earth and stayed put. In some of those minerals, tellurium impersonates sulfur, taking its place in the mineral, while in others, it acts more like a metal, bonding with sulfur to form a solid.
