By Ron B. Davis Jr., Georgetown University
In 1808, Sir Humphry Davy was on a tear; discovering new metallic elements by electrolysis of what were commonly called ‘earths’. Most of the smaller alkali and alkaline earth metals owe their names and discoveries to Davy’s technique of applying a voltage across molten mineral substances to separate their metal components from the oxygen that bound them.
Isolation of ‘Silicum’
Sir Humphry Davy’s exploits are the stuff of chemical legend, but in the midst of his remarkable success in that first decade of the 19th century, Davy also met his match while trying to isolate one of the most common elements on Earth.
In the previous century, Antoine Lavoisier had theorized that quartz, one of the most common minerals in the crust of the earth, was in fact the oxide of an undiscovered element. Quartz itself had been known since pre-history. Its common occurrence and tendency to fracture in a curved pattern that created sharp edges had made it a favorite material for the production of some of the first tools ever used by humans.
It stood to reason, then, that Davy’s techniques might successfully separate this new element from oxygen using his proven technique. And yet in this case, Davy failed. Even the most powerful batteries available to him were unable to separate the elusive element in any measurable quantity.
‘Silicium’
Not one to let a little thing like failure get in his way, Davy published his attempt in the Philosophical Transactions of the Royal Society of London in 1808, detailing his struggles with isolation of this element—after which he promptly proposed a name for the element that he was unable to collect.
Clearly believing this element to be a metal, he proposed the name ‘silicium’, a nod to ‘silex’, a name commonly used for ground flint which he had attempted to use as a source from which to isolate this element.
Studying Pure Silicon
Silicon was finally isolated by Jons Jacob Berzelius in 1824 by a different method, heating chips of potassium in a silica container.
Once pure silicon was available for study, it quickly became clear that this was a special element. As a solid, it had a dull appearance like a nonmetal, yet it was hard and fractured like a metal. It conducted electricity much better than most nonmetals, yet not nearly as well as metals themselves. Silicon was in-between a conductor and insulator—it was a semiconductor.
So why was silicon so resistant to separation from oxygen by electricity? At the time neither Davy nor Berzelius could offer a concrete explanation. In part, this is because the periodic table and the insights it offers into the properties of elements was still several decades away. In addition, in the early 1800s, it was widely believed that elements fell into two major classes—malleable, shiny, conductive metals and brittle, dull, insulating nonmetals.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Special Class of Metalloids
However, this idea that an element must be either a metal or a nonmetal had already been challenged even in the early 1700s by some chemists. Most notably, Swedish chemist Georg Brandt proposed making malleability the key characteristic of metals, regarding more brittle metals like zinc and bismuth as what he called ‘semimetals’.
But the fact that metals have a continuum of malleability makes it difficult to define a separate category of elements based on this property.
To make things even more complicated, there were those who suggested that potassium and sodium did not deserve the distinction of being metals since they floated on water, and even Berzelius himself was known to refer to nonmetals themselves as ‘metalloids’.
After decades of debate surrounding this broad system of classification of elements, the scientific community gradually has reached a consensus that metalloids are those elements with properties that are intermediate between those of metals and nonmetals.
By this modern definition, metalloids cluster somewhat neatly into a diagonal arrangement in the p-block of the periodic table. However, even today, debates over exactly which elements should belong in this special class continue.
The metalloids that are generally accepted and are most often included in this special group are boron, silicon, germanium, arsenic, antimony and tellurium.
Silicon’s Interaction with Oxygen
When we look at silicon, compared to its metallic and nonmetallic neighbors, and we consider how they each might go about achieving an octet, it begins to become clear why metalloids behave somewhat differently than the other elements.
Let’s consider a condensed periodic table showing just the s- and p-block elements.
Metals like sodium and magnesium are somewhat quick to release their sparse allocation of valence electrons. Releasing electrons leads them to become positively charged cations in their quest to achieve an octet, causing them to form ionic bonds with oxygen which we know is all too happy to accept those electrons in an ionic bond.
Davy was able to force those electrons back on to metal ions to neutralize them with his voltaic pile.
But silicon interacts differently with oxygen than it does with the alkali and alkaline earth metals. Silicon has four valence electrons that are relatively close to the positively charged nucleus of the atom, and they’re all held somewhat strongly. In fact, taking all four electrons from silicon to form an ion is too much of a challenge, even for oxygen.
So instead, silicon forms bonds to oxygen that are more covalent in nature, forming a network of covalent bonds that no amount of electrical voltage was able to break.
So Davey’s attempts to electrolyze silica, or SiO2, were likely unsuccessful because silicon dioxide is not an ionic compound. Silicon just isn’t metallic enough in character to behave like the alkali and alkaline earth metals that he so easily isolated. Instead, a sharing of electrons between the silicon and oxygen created covalent bonds that held fast even in the face of the powerful electrical potential of Davey’s batteries.
Common Questions about Sir Humphry Davy and the Metalloid Silicon
Silicon was isolated by Jons Jacob Berzelius in 1824 by heating chips of potassium in a silica container.
Silicon was special because while as a solid, it had a dull appearance like a nonmetal, it was hard and fractured like a metal. It conducted electricity much better than most nonmetals, yet not nearly as well as metals themselves. Silicon was in-between a conductor and insulator—it was a semiconductor.
Metalloids are those elements with properties that are intermediate between those of metals and nonmetals.
