By Ron B. Davis Jr., Georgetown University
Metals, nonmetals, and metalloids are different regions of the p-block. Among these, metalloids are special not just because of their famous semiconducting properties, but also because they have a tendency to take on the characteristics of other elements that they interact with, making them truly unique and useful among the elements.
Metals of the P-block
Metals of the p block are part of a group of elements called “post-transition metals” and sometimes “poor metals”. These metals are soft and have rather low melting points—think of tin or lead—yet a rather high electronegativity for metals. They are called “poor” metals in part because that high electronegativity causes hints of covalent bonding effects not commonly observed in s-block metals or transition metals. Neighboring zinc, cadmium, and mercury show similar properties, so they are sometimes considered “poor metals” as well.
Nonetheless, combine a p-block metal with a nonmetal, and just as you might expect—an ionic bond often forms. The metal transfers one or more of its valence electrons to the nonmetal. This happens in compounds like lead sulfide, which makes up the mineral galena, and tin fluoride (aka “stannous fluoride”)—a compound used in some toothpaste formulations.
But the metals of the p-block are getting farther and farther away from being able to achieve an octet by losing electrons. Just take a look at the activity series and this is clear, with d-block metals tin and lead well below the s-block metals, and therefore much harder to oxidize. These metals only grudgingly and slowly react with acid to dissolve, making lead a popular choice for plumbing in ancient times before its toxicity was fully understood.
Interestingly, there are only 20 elements out of 118 that can be reliably classed as nonmetals. Of these nonmetals, only hydrogen and helium reside outside of the p-block. In general, nonmetals hang on to their valence electrons much more tightly than metals do.
While metals often give up electrons to reach the previous octet, for nonmetals, the next octet of valence electrons is insight. Add to that the fact that for any given row, the greater number of protons in the nonmetal elements’ nuclei makes for a relatively strong nuclear charge that holds their electron cloud tighter.
So nonmetals have more valence electrons, and they hold on to them more firmly. This is what drives nonmetals to combine in covalent compounds with one another, and explains their tendency to form anions in ionic compounds with metals, where the nonmetal accepts electrons in the trade.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Remarkable Region
Perhaps the most remarkable region of the p-block is the diagonal staircase of elements called the metalloids. This region is so remarkable that many tables actually highlight these elements to set them apart from the rest of the block.
Boron, silicon, germanium, arsenic, antimony, tellurium, and polonium all belong to this class of elements. Astatine is sometimes included as well, though it is practically impossible to collect enough of this unusually rare element to reliably characterize its properties.
Stepping to the right on the table adds more nuclear charge. Stepping upward a row removes an entire valence shell of electrons and the screening they would provide the outermost electrons. This helps us to explain why the metalloids hold their valence electrons more tightly than even the poor metals, and why they occupy a diagonal region of the p-block.
The Familiar Metalloids
The most familiar metalloids like silicon or germanium have a very metallic appearance. Many of them are silver or gray and lustrous, just like metals. But the reason silicon in its pure form is used extensively in the manufacture of computer chips is because it does not conduct electricity nearly as well as genuine metals like its neighbor, aluminum, which in contrast is used in electrical wiring because of its high electrical conductivity.
Unlike their electrically conductive neighbors aluminum and gallium, silicon and germanium are semiconductors. Their nuclei are sufficiently positive and their valence electron counts are sufficiently high that these elements when pure don’t release their electrons into the conduction band nearly as well.
The Combination of Silicone with Metals or Nonmetals
The apparently wishy-washy behavior of the metalloids actually gives them unique and interesting chemistries that can be used in a variety of applications. Take the example of silicon—a brittle, shiny semiconductor when pure. Silicon can be alloyed with aluminum and other metals to create new metallic materials with beneficial properties used in aviation. In these alloys, silicon interacts through metallic bonding.
But combine silicon with nonmetals, like oxygen or fluorine, and it tends to form covalent bonds as nonmetals do. Compounds like silicon dioxide, which makes up quartz, are held together by covalent bonds.
Quartz is a network of covalent bonds between silicon and oxygen. Because the valence electrons are held up in local covalent bonds, quartz does not conduct electricity. Quite the opposite, it is used in the manufacture of electrical insulators!
Silicon can even form discrete, covalent molecules of gas. Silicon tetrafluoride, a byproduct of fertilizer preparation, consists of individual molecules consisting of one silicon atom bonded to four fluorine atoms, exactly the sort of behavior we would expect from a nonmetal.
Common Questions about the Different Regions of the P-block
Metalloids can be described as the most remarkable region of the p-block. Highlighted in many periodic tables to be set apart from the rest of the elements in the p-block region, the metalloids include germanium, boron, arsenic, silicon, antimony, tellurium, and polonium. Astatine, in some cases, is considered a metalloid as well.
Metalloids such as silicon and germanium are the most familiar in our world, looking very similar to metals. Just like metals, many metalloids are silver or gray and are lustrous; but unlike metals, silicon and germanium are not good electrical conductors and are considered semiconductors. They are extensively used for making computer chips.
Silicon, a metalloid, can alloy with other metallic elements such as aluminum to produce new metallic materials with beneficial properties. It can also bond covalently with nonmetals such as oxygen and fluorine. For example, silicon dioxide or quartz is the result of covalent bonding between silicon and oxygen.