By Ron B. Davis Jr., Georgetown University
Hypervalency, which means that an element takes on less than eight valence electrons, in the lower rows, contributes to the complexity and richness of p-block chemistry. But, the second row elements of the p-block do not display hypervalent behavior. In some cases, this can make their chemistries somewhat simpler to understand, but no less important.
The Behavior of Boron as a Second Row Element
Boron, the most electron-deficient of the nonmetals, actually can exhibit what is known as hypervalency sometimes. So although unusual, this would not be a violation of the octet rule. Since it only contains three valence electrons, boron can produce compounds in which its atoms have only six valence electrons. Some of the boron atoms in the household product borax take on this hypervalent state.
But boron can take on the additional bonds to get its octet and form some very stable compounds, as is the case when it bonds with the more electron-rich element nitrogen. These two elements can form a chemical compound called boron nitride.
By combining equal parts of elements #5 and #7, we can produce a material that behaves much like the materials combining element #6, carbon with itself. Boron nitride materials have properties similar to graphite and diamond, while being less expensive to produce.
The Comparison between Carbon and Nitrogen
With four valence electrons, carbon often forms neutral compounds with four chemical bonds. This allows carbon to form complex geometric frameworks for the millions upon millions of known organic compounds, from fuels to medicines to the biomolecules that make us who we are.
It is also this strict valence of four that gives elemental carbon some remarkably different properties compared to other nonmetals. Consider that the two best-known allotropes of carbon—diamond and graphite—both melt at about 4000° Celsius. That’s a temperature close to that on the surface of the Sun!
But look at carbon’s neighbor nitrogen. Nitrogen is a gas at room temperature. In fact, nitrogen only turns to liquid at a frigid -195° Celsius—colder than the dark side of the moon! And solid nitrogen can only exist below -210° Celsius. We have to go to the moons of Uranus or the dwarf planet Pluto to find the closest naturally occurring solid nitrogen.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Why the Melting Point of Carbon and Nitrogen Differ
So how can two elements so close to one another on the table have such different melting points? The answer lies in the strength of covalent bonds and the valence of these two elements.
Nitrogen has five valence electrons, leading it to seek out three chemical bonds to satisfy its octet. A nitrogen atom can do this by forming a triple bond with another nitrogen atom. This creates a concerted diatomic molecule of nitrogen—one that can easily drift away from others into the gas phase.
But carbon needs a fourth bond to satisfy its octet. But there isn’t enough space between those carbon atoms to hold that fourth bond. Instead, carbon adheres to the octet rule by bonding to more than one carbon atom through single or double bonds, creating vast networks of covalent bonds in materials like graphite and diamond.
This locks carbon atoms in the solid phase. For carbon to melt, the powerful and stable covalent bonds holding the network of atoms together must be broken—a process that takes enormous energy.
The Behavior of Oxygen, Fluorine, and Neon
Nitrogen’s other neighbor, oxygen, on the other hand, behaves quite similarly to it when it comes to melting and freezing points. This is because oxygen has six valence electrons, meaning that it will typically seek out two bonds forming a double bond with itself. This leads to discrete molecules that behave much like nitrogen in many ways.
But the double bond in that oxygen molecule is somewhat easier to break than the triple bond in a dinitrogen molecule, because fewer electron pairs are involved in the chemical bond. That’s why oxygen gas is more reactive than nitrogen gas. O2 can support combustion, and our bodies can use molecular oxygen to release biochemical energy in the foods we consume. Meanwhile, atmospheric nitrogen is so stable that our bodies make no use of it whatsoever.
Taking one more step across row 2 we encounter fluorine, which forms only a single bond in its elemental state. Fluorine’s single bond in its diatomic state is easily broken in many chemical reactions, including reactions with the tissues in the skin and the moisture in the lungs.
So, it owes its famous reactivity to the fact that each fluorine atom is just one electron shy of an octet. Of course, neon’s closed valence shell results in a monatomic substance that is practically unreactive.
Common Questions about the Second Row Elements of the P-block
Because of the strong network of covalently bonded atoms of carbon, this element needs a temperature of around 4000° Celsius to be melted. On the other hand, carbon’s neighbor in the second row element of the p-block region, nitrogen, is a gas at room temperature. To turn this gas into a liquid, we need a temperature even colder than the dark side of the moon at -195° Celsius.
These two neighboring second row elements differ in their melting points because of their valence and the strength of their covalent bond. Nitrogen has five valence electrons, so it forms triple bonds with other nitrogen atoms making a concrete diatomic molecule that can drift away into the gas phase. But carbon, with the valence of four, can form compounds with four chemical bonds. This gives it the ability to form complex and vast geometrical networks of covalently bonded atoms in materials such as diamond and graphite.
The second row elements of the p-block consist of boron, carbon, nitrogen, oxygen, fluorine, and neon. Boron is one of the most electron efficient nonmetals, with three valence electrons that sometimes become hypervalent. Carbon has four valence electrons, nitrogen has five, and oxygen has six electrons in their valence shell. Fluorine needs only one electron to satisfy its octet which makes it absolutely reactive. Last, but not least, neon has eight valence electrons, making it completely unreactive.