By Ron B. Davis Jr., Georgetown University
The nonmetal elements of the p-block are complex both in the allotropes they can form and the roles they can play in compounds. These elements—carbon, phosphorus, sulfur, and selenium—sometimes collectively referred to as the polyatomic nonmetals, prefer to form larger, more complex elemental molecules using many atoms of the same element.
Complex P Orbital of Carbon
Carbon is a small atom, with its second electron shell as its valence shell. It can make three bonds to another carbon, much like nitrogen. But when we examine the geometry of the p-orbitals carefully, we see that the final p orbital is not aligned properly to form an additional bond the way the others are.
It has to reach out to another atom to form its fourth bond. And now that atom must find other atoms to satisfy its octet in a never-ending quest to achieve octets for all carbon atoms in the sample.
Carbon Allotropes and Its High Melting Point
Carbon commonly takes two structures. One is graphite which is an enormous sheet of carbon atoms each bonded to three others in a planar arrangement. The other is diamond where each carbon atom is covalently bonded to four others, producing a three-dimensional network of atoms.
Hence, to melt carbon we have to literally break all of the covalent chemical bonds in the vast network, depriving carbon atoms of their octets in the process.
This so-called network-covalent structure of graphite and diamond, resulting from carbon’s inability to form a diatomic molecule, gives them extremely high melting points. Graphite is used in control rods in the cores of nuclear reactors. Diamond is used as an insulator in high-temperature applications.
Carbon’s high melting point also helps to produce coal.
Valence of Sulfur, Selenium, and Phosphorus
The valence of sulfur, selenium and phosphorus allows them to make multiple bonds. However, the larger size of their atoms makes for poor p-orbital overlap, which results in much weaker multiple bonds that must form in order for small, diatomic molecules to form.
The atoms of selenium, phosphorous, and sulfur are forced to satisfy their octets by bonding to multiple atoms in their elemental form. These elements tend to form large, discrete molecules. This P4 molecule is common in phosphorous allotropes. The S8 and Se8 molecules are often found in sulfur and selenium samples.
These larger molecules have greater Van der Waals forces than their neighbors to the right, sticking to one another better than the diatomic nonmetals or noble gases do, which explains why their melting points fall somewhere between diatomic nonmetals and carbon.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Polyatomic Phosphorus
Phosphorus is another nonmetal that has a rich and complex set of allotropes. But, phosphorus owes its polyatomic structures, not to its valence, but to its size. Di-phosphorus is extremely unstable since the larger phosphorus atoms cannot achieve the good p-orbital overlap necessary to form strong multiple bonds.
When phosphorus is heated to extreme temperatures—above about 800° Celsius—it succumbs to the forces of entropy and decomposes into molecules of P2, with two phosphorus atoms joined by a triple bond similar to that of nitrogen’s diatomic structure.
Phosphorus Allotropes
The most thermodynamically stable form of pure phosphorus is a dark gray solid composed of large puckered sheets of interconnected phosphorous atoms, each bonded to three others by covalent single bonds. In this form, called black phosphorus, it’s resistant to oxidation and conducts electricity, behaving much more like graphite than nitrogen.
But phosphorus is more commonly encountered as one of its other, more colorful allotropes: red phosphorus or white phosphorus.
Like black phosphorus, red phosphorus contains long chains of bonded atoms, but they are amorphous. Like black phosphorous, red phosphorous is not highly reactive, and it is this amorphous allotrope that is commonly used in chemical reactions for safety purposes.
White phosphorus is composed of small, discrete P4 molecules and is much more reactive. The discrete molecules of this form of phosphorus are so reactive that the phosphorus actually glows as it reacts with oxygen when exposed to air. Its extreme reactivity has made it a commonly used incendiary agent in military munitions.
Properties Of Sulfur
Another nonmetal element that can form polyatomic molecules is sulfur, despite being located below oxygen in the table. For the reasons similar to phosphorus, being on the third row of the table means sulfur is a larger atom and therefore it less prone to forming multiple bonds. But being from oxygen’s group, sulfur by-and-large only has two bonds to offer.
A series of sulfur atoms with two bonds each could form a chain, but sulfur atoms at the end of a chain would not be able to satisfy their octet.
This makes sulfur especially prone to form ring-like molecules, allowing every sulfur atom to have two bonds. Such rings can have varying numbers of atoms in them, all connected to one another through single bonds to achieve an octet of valence electrons.
The most common of these sulfur rings is the S8 molecule. It can exist as three different polymorphs that differ only by the arrangement in which the S8 molecules are packed together in the solid.
But even sulfur, just like phosphorus, shows its nonmetal side when heated to high temperatures. At about 740° Celsius, sulfur finally gives in and forms a vapor of S2 molecules joined by a double bond, just like oxygen.
Properties of Selenium
Selenium can also form cyclic molecules of eight atoms. But selenium’s superpower is an ability to form loops that can be hundreds, or even a thousand, atoms in size.
Its most common allotrope is amorphous red selenium. But if red selenium is gently heated, it’s also possible to obtain more structured allotropes of black and grey selenium.
Selenium is also special for its relationship with light. Selenium is both photovoltaic, meaning it can generate electricity from light, and photoconductive, meaning that exposing it to light changes its ability to conduct electricity.
These properties have made selenium useful for light meters once used in camera technology and in recent years have made it a promising ingredient in new solar cell designs.
Common Questions about Properties of Polyatomic Nonmetals
Carbon takes two structures. One is graphite which is an enormous sheet of carbon atoms each bonded to three others in a planar arrangement. The other is diamond where each carbon atom is covalently bonded to four others, producing a three-dimensional network of atoms.
Sulfur shows its nonmetal side when heated to high temperatures. At about 740° Celsius, sulfur forms a vapor of S2 molecules joined by a double bond.
Selenium is both photovoltaic, meaning it can generate electricity from light, and photoconductive, meaning that exposing it to light changes its ability to conduct electricity.
