By Ron B. Davis Jr., Georgetown University
In the top right corner of the table are the nonmetal elements. Here we find elements that exhibit new tendencies, such as covalent bonding to reach an octet. Some of the larger nonmetals routinely even exhibit hypervalency, making their behavior even more diverse. Nonmetals can be further divided into a few sub-groups.
The Noble Gases and Halogens
The most distinct is the noble gases—those elements with a completely full p-subshell, and consequently a completely full octet in their valence shell. These gases—neon, argon, krypton, xenon and radon—all feel no need to interact with other elements, including other atoms of themselves! This helps to explain why the noble gases so strongly prefer the vapor phase, having some of the lowest boiling points on the table.
Group 17, the halogens, form discrete diatomic molecules when pure, since each only needs a single covalent bond to achieve an octet. Pure fluorine, chlorine, bromine, or iodine each form just one type of molecule when found in nature—a molecule with two atoms joined by a single bond. But the carbon, nitrogen and oxygen group nonmetals have more complex bonding behavior that can lead to more variety.
Different Allotropes of an Element
The group 16 nonmetals oxygen, sulfur and selenium can form two bonds per atom. This gives them the ability to form molecules consisting of chains or even loops with varying numbers of bonded atoms. such molecules consisting of the same element, but varying size and shapes are called allotropes.
Oxygen, for example, forms a diatomic molecule that we breathe here on the surface of the Earth. In the oxygen atom, both bonds join the same two atoms, producing what is known as a double bond. This creates the O2 molecule, the most common form of oxygen in our environment—the one you are breathing right now.
But there is another allotrope of oxygen called ozone, which is a TRI-atomic molecule that absorbs harmful radiation in the upper atmosphere, but also causes significant pollution problems if it is allowed to build up in our surface environment. In ozone, the central oxygen atom is bonded to two different oxygen atoms, producing a short chain of three atoms.
Sulfur molecules can get even larger, forming molecular rings with as few as five atoms or as many as eight. And selenium can form cyclic molecules with several hundred or even a thousand atoms per molecule. But with only two bonds per atom allowed, there can be no branching or higher order geometry in a molecule of a group 16 nonmetal. Anything more complex than a ring or a long chain isn’t possible.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Varied Three-dimensional Molecules
Group 15 elements, by contrast, can form three bonds per atom, this opens the door to three-dimensional molecules with a wide variety of shapes. Phosphorus, for example, forms molecules as small as four atoms in an allotrope called white phosphorus, but also in vast networks of covalently bonded atoms in different geometries, called black and red phosphorus.
Even nitrogen, though always found in nature as highly stable N2 molecules joined by a powerful triple bond, can theoretically form complex allotropes with really fun names like hexazine and octaazacubane. But these allotropes of nitrogen would be much less stable than N2, and to-date have only been the subject of theoretical and computer modeling studies.
Carbon: Very Special
But carbon, the only nonmetal in group 14, really takes the cake when it comes to allotropes. Being the only nonmetal with four valence electrons makes pure carbon very special. This means that carbon atoms can form up to four bonds with one another in search of their octet.
In our environment, pure carbon commonly takes on two different allotropic forms, both of which involve huge networks of strong covalent bonds. The most notable of these are one of the hardest natural materials on Earth—diamond, and soft, dark graphite, both of which contain complex networks of atoms in different bonding patterns and arrangements.
From the individual atoms of noble gases to the complex allotropic molecules of carbon, when it comes to diverse behavior, the p-block delivers.
Common Questions about Nonmetals and Their Bounding Behavior
Some nonmetals can create more than one bond per atom, making them able to form and create molecules consisting of many repeated chains or even loops with different numbers of bonded atoms. These kinds of molecules that consist of the same element, but are different in shapes and sizes are called allotropes. For example, ozone is one of the allotropes of oxygen and diamond is an allotropic form of carbon.
Nonmetals in group 15 have the ability to form three bonds per atom, meaning they are able to form three-dimensional molecules with varying shapes. For instance, atoms of phosphorus can form small molecules with only four atoms per allotrope which are called white phosphorus. On the other hand, red and black phosphorus is made up of a huge network of covalently bonded atoms.
Carbon, the only nonmetal appearing in group 14 of the periodic table, is also the only nonmetal having atoms with four valence electrons. Therefore, it can reach its octet by covalently bonding with four other carbon atoms, and that is why carbon compounds appear in huge, complex, and vast networks of strong covalent bonds. Diamond and graphite are good examples of these compounds in nature.
