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.
