8.2 Molecular Compounds
Properties of
molecular
substances
Properties of molecular compounds vary widely compared to those of ionic compounds:
A molecule is
the building
block of a
molecular
substance
The properties of molecular substances depend on
two things: the structure of the individual molecule,
including the types of covalent bonds formed within
the molecule, and the attractions between molecules.
Take water for example. The smallest piece of water
you can have is a single water molecule. However,
when we think of water we imagine a drop, or a cup,
or an ocean. One cup of water is made from
approximately eight million billion billion (8×1024)
water molecules. They bounce off of each other,
constantly in motion, forming no particular
crystalline organization when in liquid form. As ice,
water can form an organized crystal structure, and in
that form it has the property of being hard and brittle
like an ionic compound. The great variety of
structures possible with molecular substances is what
gives them their great variation in properties.
Molecules can
be classified
into different
categories
First we will look at the different types of molecular building blocks. Let’s study how
their interactions affect the overall properties of the substances made of those molecules.
Molecules are typically small, medium or large. Within the “large” category there are
polymers and network covalent substances.
• Some are hard and brittle, while others are flexible, or soft and mushy.
• They could be solid, liquid or gas at room temperature.
• Their boiling points vary from -253°C to over 4800°C.
• Most do not conduct electricity well regardless of their state of matter.
• When dissolved in a solvent, they don’t typically conduct electricity.
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Section 8.2 Molecular Compounds
Small molecules
Most small
molecules are
liquids or gases
at room
temperature
Small molecules are those that are constructed from about a dozen
atoms or less. There is no specific number of atoms that puts a
molecule in the “small” category, but substances made from small
molecules tend to have some similar properties. They are almost
always gases or liquids at room temperature and include some of
the most important molecules related to the functioning of our
entire ecosystem. Often they are over the counter drugs, such as
acetaminophen, which in its pure form is a liquid.
The lower
density of ice,
compared to
liquid water,
made the
evolution of
human life
possible
Water is a good example. A single polar water
molecule is made from only three atoms, but this
molecular substance is essential for life. It is how
the rest of the molecules in our body organize
themselves. When it freezes, the solid form is
less dense than the liquid form, something
almost unique to water. If ice were more dense,
then lakes, ponds, and even oceans could freeze
solid in cold weather, killing all higher life
forms, and preventing the evolution of anything
beyond the simplest of creatures.
Most common
gases are small
molecules
Carbon dioxide, and oxygen fall into the small molecule category as well. These are all
gases at room temperature, and it is not by chance that they are all non-polar. Carbon
dioxide is also a greenhouse gas, but it is also crucial for the lives of plants, which use the
carbon to build up their physical structure, and as a temporary chemical storage for the
light energy absorbed by their leaves. Typically it is the polarity of the molecule that
makes the difference between it being liquid or gas at room temperature.
Some other small molecules you may be familiar with are ethene, which is a non-polar
gas given off by ripening fruit, and ethanol which is almost the same size but polar and is
liquid at room temperature. When fruits and other sugar-containing material ferment, this
substance is produced, making what is commonly called alcohol.
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Medium-sized molecules
Liquid or soft
solids
Medium sized molecules are those that are typically
formed from more atoms than small molecules, but
not much more than 100 atoms. As with small
molecules, there is a significant variety of molecule
types that fall into the “medium” category, but they
tend to share some properties. Typically, they are
liquids or soft solids at room temperature, and often
fall into one of two official chemical categories:
lipids or long chain hydrocarbons.
Lipids
Lipids are typically fats or steroids, molecules primarily made from carbon and
hydrogen, but may include small numbers of other elements. Depending on the size and
shape of a fat molecule, the substance made from those molecules might be liquid or
solid at room temperature, for example vegetable oil or butter. Lipids are typically nonpolar and insoluble in water.
Steroids are
another type of
lipid
Steroids also fall into the lipid category, but have a larger
percentage of non-hydrogen and carbon atoms. Most have
four rings of carbon atoms, and could function as
hormones, drugs, vitamins or poisons. Anabolic steroids,
which are used as performance enhancing drugs, all mimic
the shape and function of testosterone, a natural steroid.
Hydrocarbons
Hydrocarbons are molecules formed purely from long chains of hydrogen and carbon
atoms. They are very similar to fats, except they lack the typical oxygen atoms that are
part of the fatty acids produced in living organisms. Medium-sized hydrocarbons tend to
form soft solids like petroleum jelly, and ones made from even larger numbers of atoms
tend to form harder substances like wax.
lipid - a molecule that typically falls into the category of fat or steroid.
steroid - a molecule with four carbon rings that is biologically active as either a
hormone, vitamin, drug or poison.
hydrocarbon - a molecule made almost entirely from carbon and hydrogen atoms.
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Section 8.2 Molecular Compounds
Polymers made from a single monomer
Polymers and
monomers
The prefix “poly” means “many,” which is appropriate for the
kinds of molecules we call polymers, because polymers are
made from many smaller molecules. The smaller molecules
that covalently bond together to make polymers are called
monomers. All of the plastic in the world is made of
various kinds of polymers. The recycling numbers on bottles
and packages indicate which kind plastic/polymer the object
is made from. Number 3, for example, is polyvinyl chloride
(PVC), which is made into pipes and siding. Many vinyl
chloride monomers bond together to form long chains,
making individual polyvinyl chloride molecules that consist
of hundreds or thousands of atoms.
Polymers can
be synthetic
Plastics are synthetic polymers, typically made in labs and factories.
However, many natural polymers exist, made by living organisms.
The simple sugar glucose is a key monomer for making two
different natural polymers: starch and cellulose. By bonding the
glucose monomers together in a slightly different way, you either get
something we can digest, starch, which is found in many foods like
bread, pasta and potatoes, or something we can’t digest, cellulose
which is what wood, paper and most plant material is made from.
polymer - a molecule made from bonding many small molecules together.
monomer - a small molecule which is a building block of larger molecules called
polymers.
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Polymers made from multiple monomers
A single
polymer can be
made from
more than one
type of
monomer
Each of the polymers we saw on the previous page were
all made from a single type of monomer. Sometimes
more than one monomer is used. DNA is a polymer that
uses four different monomers that biologists call
“nucleotides.” Proteins are constructed from up to 20
different monomers called amino acids. To distinguish
between these two types of polymers, we use the term
homopolymer to refer to polymers made from only
one type of monomer, while copolymer describes
polymers made from multiple types of monomers.
DNA: the code
for building
proteins
DNA is the molecule that provides the code
necessary to build proteins. The proteins are built
by adding one monomer (amino acid) at a time.
The order of the amino acids in the protein
polymer is based upon the order of the monomers
(bases) in the DNA. Once the full protein is
made, various parts of the molecule attract to
itself, causing it to form into a particular shape,
giving the protein a specific function.
homopolymer - a molecule made from repeatedly bonding the same monomer
together.
copolymer - a molecule made from repeatedly bonding more than one type of
monomer together.
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Section 8.2 Molecular Compounds
Network covalent molecules
Network
covalent
Usually, when we think of molecules we think of things so
small that we can’t even see them with a normal
microscope. However, when you see sand, it is likely that at
least some of the individual sand grains are actually made
from a single individual molecule. Sand is usually a
mixture of many different minerals, but the most common
one is quartz, which is made from silicon and oxygen. Each
silicon is covalently bonded to four neighboring oxygen
atoms, and each oxygen atom is bonded to two neighboring
silicon atoms. Since every atom in quartz is covalently
bonded together, the entire grain can be considered a single
molecule. Molecules in which large numbers of atoms are covalently bonded in an
interlocking way are called network covalent.
Carbon
Carbon is special because it can form many different kinds of network covalent
substances, some of which have amazing properties and were among the first substances
to bring about the era of nanotechnology. Below are several forms of carbon. Diamond
and graphite are commonly found in nature, while buckyballs and nanotubes are
primarily synthesized in labs. Buckyballs do occur in nature, but in only small amounts.
network covalent - a type of large molecule, usually made from hundreds to billions
of atoms, in which each atom is covalently bonded to multiple neighboring atoms,
forming a web of connections.
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Chemical formulas of covalent compounds
Molecular
substances
have a smallest
piece
With ionic compounds, the formula indicates the ratio of
ions in the compound. This makes sense, because there is
no specific boundary between each “piece” of the ionic
crystal. In other words, there is no specific group of ions
within the larger structure of the substance that we would
isolate for any special reason. In molecular substances, like
water, or ethanol, there is a “smallest piece” that is
significant - a single molecule of that substance.
Empirical
formulas
Network covalent substances are treated more like ionic
substances in which the ratio of elements is used. This is
because the large number of covalently bonded atoms forms
a single unit, just like the large number of bonded ions do in
ionic substances. For example, the formula for quartz is
SiO2, indicating that in a grain of typical quartz-based sand,
the ratio of silicon to oxygen will be 1:2. A formula that
describes the simplest ratio of elements in a substance is
called an empirical formula.
Molecular
formulas
Chemical formulas for most molecular substances indicate
the exact type and number of each atom in a single
molecule of that substance. This is called a molecular
formula. The elements in a molecular formula are ordered
alphabetically, unlike in ionic compounds where the
positive ion is listed first. Sometimes the empirical formula
is the same as the molecular formula, as is the case for
water (H2O), but sometimes the molecular formula is not
the empirical formula, as is the case for benzene (C6H6).
Write the molecular formula for each of the molecules below:
Asked:
Use the molecular structures to determine the molecular formula.
Relationships: The formula should reflect the exact number and type of atoms in an
individual molecule of the substance.
Solve:
a) C2H2
b) NH3
c) C2H5O
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Section 8.2 Molecular Compounds
Naming for simple binary molecular compounds
Organic
molecules
Molecular substances vary so much in their basic
structures that different naming systems are used
depending upon the molecule. Organic
molecules (those made primarily of carbon,
hydrogen, oxygen, and the halides) have an
entire, very complex naming system dedicated to
that class of substances. In this chapter we will
look at naming simple binary molecular
substances, those made from only two elements.
Naming simple
organic
molecules
Naming these simple inorganic molecules is much easier than
with ionic substances, because there is a basic two-step
procedure for creating the name:
1.
2.
Write down the name of the compound as if it were a
simple binary ionic compound.
Add prefixes to each name which tell you how many
of that atom are in the molecule (Note: If there is
only one atom of the first element in the molecule
you can leave out the “mono” prefix from the name.).
Write the name for each of the following formulas: N2O4, S2F10, SO3
Asked:
Name the formulas above.
Given:
Three formulas, and a table of prefix names.
Relationships: The name of the formula is constructed from the simple ionic name with
prefixes before each element, indicating the number of each atom.
Solve:
N2O4 = dinitrogen tetraoxide
S2F10 = disulfur decafluoride
SO3 = sulfur trioxide
empirical formula - the simplest ratio of atoms in a substance.
molecular formula - the exact number and types of atoms in a molecule.
organic molecule - a molecule primarily made from carbon and hydrogen, but often
with some oxygen, nitrogen, one of the halides, or some other non-metal atoms.
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