Video: Network Covalent Structures

In this video, we will learn how to recognize network covalent structures and describe their general properties.

16:13

Video Transcript

Network Covalent Structures

What do diamond, graphite, and quartz have in common? Well, if we zoomed in on the arrangement of their atoms, we would find a long, continuous, repeated-branching network of covalent bonds. We call these kinds of substances network covalent structures. In this lesson, we will learn how to recognize network covalent structures and describe their general properties.

Network covalent structures are also called giant covalent structures or covalent network solids. They are compounds or elements where the atoms are held together by a continuous network of covalent bonds. We can see this structure in the diagram below. In diamond, carbon atoms are held together by a network of covalent bonds. Because the network has many more bonds than a typical molecule, we sometimes refer to network covalent structures as macromolecules.

Two different macromolecules of, say, diamond might have a different number of atoms within them. Because of this, the chemical formula for a network covalent structure is not the number of each type of atom in the macromolecule. The chemical formula instead gives the atoms present in the repeated unit that makes up the network. For example, graphite is made up of a repeated network of carbon atoms. So, its chemical formula is C. Quartz, on the other hand, is made up of a repeated network of silicon atoms and oxygen atoms in a one-to-two ratio. So, its chemical formula is SiO2. Note that graphite and diamond are both made up of carbon atoms. However, the arrangement of these carbon atoms is different in each. This leads to different substances with different physical properties.

To put our understanding of network covalent structures into context, let’s compare them to simple molecular structures. Simple molecular structures will have a set number of covalent bonds. For example, all water molecules have two covalent bonds, one bonding each of the two hydrogen atoms to the central oxygen atom. We call these units molecules. The chemical formula is simply the makeup of each molecule. For example, water, H2O, is a molecule made of two hydrogen atoms and an oxygen atom. Carbon dioxide, CO2, is another simple molecular structure. Visualized in the diagram below, we can see the distinct molecules of water, each with one oxygen and two hydrogen atoms.

There are attractive forces between the molecules, for example, hydrogen bonds, seen here as the dotted lines that connect the hydrogen and oxygen atoms in different molecules. However, these intermolecular forces, or forces between molecules, are much weaker than the intramolecular forces, or the forces within the molecules. In other words, the covalent bonds holding together a network covalent structure, such as the carbon-to-carbon bonds in diamond, are much stronger bonds than the weaker intermolecular forces holding together a simple molecular structure, for example, the hydrogen bonds holding together the molecules in a sample of water.

In fact, the bond strength of network covalent structures explains many of the physical properties that emerge. Network covalent structures have high melting points. Since each atom is held in place by multiple strong covalent bonds, it takes a lot of energy to loosen them and create a liquid. It is easier to loosen the network of a simple molecular structure like water, which is held together by weaker intermolecular forces.

Network covalent structures are also quite hard. Substances like diamond can withstand lots of force without breaking. Each carbon atom is bonded rigidly to four nearby carbon atoms. Since the atoms are held in place by bonds in multiple directions, there’s not a lot of freedom of motion when a force is applied, so it takes a great amount of energy to break apart the diamond. When enough force finally is applied, the structure will break apart instead of bend, which makes these structures very brittle.

Lastly, most network covalent structures don’t conduct electricity. Electricity is the flow of charged particles, often electrons. Since the valence electrons of diamond are held in place between atoms of carbon, they are not free to flow through the substance. One exception to this rule is graphite. The arrangement of electrons in the network of carbon atoms that make up graphite leaves some electrons free to flow through the substance, allowing electricity to conduct through the material. However, in other covalent network structures, the valence electrons are held in place by rigid covalent bonds, an arrangement that does not allow for electricity to conduct.

Interestingly enough, many of these physical properties are also shared by ionic compounds. In the structures of ionic compounds, sometimes called giant ionic structures, there are strong, multidirectional, electrostatic attractions between the positive ions and the negative ions. While the main difference between ionic compounds and network covalent structures is the presence of covalent versus ionic bonds, there are many similarities as well.

In both structures, the particles are difficult to loosen, which raises the melting point. The multidirectional arrangement of the bonds makes it so that both types of substances can withstand lots of force without breaking. But when they do finally break, they do not bend, making them hard and brittle. And while it is true that for both ionic compounds and network covalent structures, the electrons are held in place preventing them from conducting electricity, ionic compounds do conduct electricity when they are melted or dissolved.

This is because the charged particles in the form of the ions that make up the ionic compound are released from their rigid structure and are free to flow through the material. Electricity is the flow of charged particles, so melting or dissolving an ionic compound provides charged particles that can flow. Network covalent structures are not soluble in water and generally do not conduct electricity even when melted. So, that is a big distinction between these two types of substances.

Now that we’ve learned a little bit about the structure of network covalent solids, their physical properties, and how they compare to other types of substances, let’s do some practice problems to review.

Which of the following properties can generally be used to differentiate a molecular solid from a covalent network? (A) Electrical conductivity, (B) brittleness, (C) color, (D) melting point, or (E) chemical reactivity.

This question is asking us to find a key difference between simple molecular structures made up of a set number of covalent bonds and covalent network structures made up of a continuous network of covalent bonds. In order to find the property that differentiates these two, we need to find the property that is both distinct between the two and consistent within each type. In other words, we wanna make sure that all simple molecules consistently have a certain physical property, and we wanna make sure that all covalent network solids have the opposite physical property.

Let’s take a look at the choices one by one. Unfortunately, there’s no consistent color pattern to simple molecules or covalent networks. There are a variety of colors of substances of each type, so we can’t rely on color to differentiate these two types of solids.

We can also eliminate brittleness as an answer. While covalent networks are brittle, simple molecules with covalent bonds can be brittle or they can be more soft and flexible. Since some simple molecules are brittle, we can’t use brittleness to differentiate between them and covalent networks.

There’s no difference in electrical conductivity as well. Neither simple molecules nor covalent networks allow for electricity to conduct through the substance as there are no free charged particles to flow. Since there is no difference here, we can eliminate (A) as an answer.

There’s not a clear pattern for choice (E), chemical reactivity, either. Covalent network solids are generally unreactive, while simple molecules can be reactive or not reactive. For example, the molecular solid glucose, C6H12O6, is relatively reactive, whereas the molecular solid dry ice, the solid form of CO2, is relatively unreactive. So, we can eliminate choice (E).

The last remaining choice, and the correct choice, is choice (D) melting point. Molecular solids have low melting points, while covalent network solids have quite high melting points in comparison. Substances consisting of simple molecules are held together by weak intermolecular forces. In the diagram here, the strongest intermolecular force is the hydrogen bonds holding together the water molecules pictured.

On the other hand, network solids are held together by relatively stronger covalent bonds. These bonds are an example of intramolecular forces or forces within the molecule. The weaker forces in simple molecules lead to a lower boiling point because it takes less energy to separate the particles of the substance. On the other hand, the strong covalent bonds found in network solids rigidly hold together the atoms of the substance. It takes a lot more energy to loosen the atoms held in this rigid way. So, these substances have relatively high melting points.

Looking at some examples confirms this pattern. Molecular substances like propane and water tend to have melting points below room temperature, whereas covalent network solids like diamond and graphite have melting points in the sweltering thousands of degrees. It’s likely impractical to reach this temperature with a simple laboratory setup. So, when comparing the melting points of different substances, it’s easiest to use their referenced values from a textbook.

The property that can generally be used to differentiate a molecular solid from a covalent network is the melting point.

At room temperature and pressure, the most stable form of sulfur is the simple molecule S8. However, heating the material to a high temperature can produce continuous chains of sulfur atoms. Which term best describes the type of structure displayed by high-temperature sulfur? (A) Ionic, (B) simple molecular, (C) giant covalent, (D) metallic, or (E) atomic.

This question is asking about the type of structure displayed by high-temperature sulfur. While they acknowledge that sulfur can form simple molecules, the high-temperature version that they’re talking about involves continuous chains of sulfur atoms. The key words here are “continuous chains.” Which of these five choices describes the structure with continuous chains of atoms bonded together? That’s choice (C), giant covalent structures.

Giant covalent structures involve continuous networks of covalent bonds, exactly what they are describing in the question. To be thorough, let’s eliminate the other choices from consideration as well.

It’s worth noting that sulfur on the right-hand side of the periodic table is a nonmetal, as it forms a sulfur two minus ion. This allows us to eliminate both ionic and metallic from consideration. In the case of metallic, sulfur is a nonmetal, so it can’t form metallic bonds. In the case of ionic, while sulfur can form ionic compounds, there is no metal to provide a positive ion to form the other half of the ionic bond in this situation.

It is not a simple molecular structure, either, because simple molecular structures are not continuous. Continuous implies that the chains go on and on with an indefinite size, whereas simple molecules have a definite size. Lastly, atomic structure is too general of a term to best to describe the structure displayed here. While we could describe the giant covalent network described here as an atomic structure, we could also describe all chemical compounds as atomic structures. So, it’s not a narrow enough term to specifically describe the structure displayed here.

In the end, the key piece of information to recognize is that a continuous chain of atoms suggest a giant covalent structure. Note that giant covalent and network covalent are different names for the same structure, and we can use them interchangeably.

So, which term best describes the type of structure displayed by high-temperature sulfur? That’s a giant covalent structure.

Now that we’ve done some practice problems, let’s review the key points of the video. Network covalent structures involve atoms held together by continuous networks of covalent bonds. Examples of network solids include diamond, graphite, and quartz. Network covalent structures are distinct from simple molecules. Simple molecules have a distinct size, whereas network covalent structures have a continuous branching network of atoms.

Network covalent structures are also distinct from ionic compounds. While both of these structures involve continuous networks of particles, ionic compounds are made up of ionic bonds as opposed to the covalent bonds found in network covalent structures.

Network covalent structures have consistent physical properties: namely, that they have high melting points, they are hard, they are brittle, and in general they do not conduct electricity. One exception to this is graphite, which can conduct electricity due to the presence of delocalized electrons. And the physical properties are due to the strong and rigid bond structure. The microscale properties of the network explain the macroscale properties of the substance.

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