|Why is diamond harder than coal if they are both
made up of carbon?|
|Question Date: 2014-08-08|
You've touched upon a really interesting material.
Carbon is a material with many allotropes (i.e.
different structural forms of the same material).
Other forms of carbon you may be familiar with are
graphite and carbon nanotubes. Carbon has four
valence electrons, which means it tends to form
sp3 and sp2 hybridized bonds. These bonds are
covalent bonds, which are very strong. So why
isn't coal as hard as diamond?
It has to do with bonding.
What kinds of structures are possible in carbon
Coal is indeed composed of mostly carbon, but
there are many other substances also present. In
fact, coal is a huge hydrocarbon, shown
You can see there are benzene rings and linear
chains of carbon spewed among many functional
Contrast that with diamond, which is exclusively
composed of C-C covalent bonding in a periodic
here for image
As you can tell, the packing of this hydrocarbon
isn't as good as it is in diamond. It takes less
energy to disrupt the structure in coal than it
does in carbon because it is easier to break
secondary bonding that occurs within the
hydrocarbon itself. These include Van de Waals
forces and hydrogen bonding. There are also
just more covalent bonds to break, which adds to
the hardness of diamond.
But having many types of bonding can also be
advantageous. If you contrast graphite and diamond
(above), even though both are essentially C-C
bonds, their mechanical properties are very
different. Graphite is composed of sheets of
carbon that are held loosely by weak Van de Waals
forces. The weak interaction between layers is
what makes graphite ideal in pencils for writing
and as a lubricant- when you're using graphite,
you're just pulling apart all these layers!
The structure of graphite is also easily
translated into carbon nanotubes; you can think of
carbon nanotubes as rolled up sheets of graphene
(a single sheet in graphite). Carbon nanotubes are
an exciting area of active research. Various forms
of carbon nanotubes have been found to be harder
than diamonds, more conductive than copper, and
stronger than steel!
Hope this helps!
Thank you for your question! Carbon, as with many
elements, can arrange its atoms into several
different geometries, or "allotropes." In pure
diamond, every carbon atom is covalently bonded to
exactly 4 other carbon atoms in a very specific
and energetically favorable geometry. The diamond
cannot be broken or scratched unless many covalent
bonds are broken, which is difficult to do. In
another common allotrope, graphite, every carbon
atom is covalently bonded to only 3 other carbon
atoms, and the atoms are arranged in sheets that
are not covalently bonded to each other. The
sheets can be broken apart easily, ultimately
meaning that graphite can be easily scratched.
Coal is composed of particles of different
allotropes of carbon, and some "amorphous carbon,"
which has no defined geometry in its atomic
structure. Without a continuous network of
covalent bonds, coal is easily scratched (i.e. it
is not hard).
First, you need to understand that there are many
different types of coal – lignite, bituminous,
anthracite, graphite, etc. – that vary in the
amount of carbon present (some types have other
elements like hydrogen and sulfur). For the
purpose of this question, "coal" is referring to
"graphite", which is made up almost entirely of
carbon atoms (like diamond).
Looking at a dirty piece of coal and a
beautiful diamond, it's hard to believe they are
composed of the same main element – carbon. Coal
and diamond are two examples of carbon allotropes,
where the carbon atoms are bonded together in
different configurations. These structural
differences result in very different material
properties, such as hardness. According to the
Mohs Hardness Scale, numbered 1 (softest) to 10
(hardest), coal is a 1 or 2 whereas diamond is a
The carbon atoms in coal are arranged in 2D
sheets, where each carbon atom is bonded to 3
other carbons to form hexagonal rings. These
sheets are held together by weak bonding forces
called van der Waals forces, which is why coal is
so soft (think: graphite in your pencil). On the
other hand, each carbon atom in diamond is bonded
to 4 other carbons, forming a rigid, 3D
crystalline lattice. These strong covalent bonds
between the carbon atoms give diamond its superior
Hope this helps!
They have different chemical structure. In
diamond, each carbon atom is covalently bonded to
another carbon atom in a tetrahedral arrangement,
making a uniform, tight lattice in space. Coal
actually isn't a mineral (and isn't pure carbon),
but graphite is. Graphite however consists of
sheets of carbon atoms with a floating double bond
between the different atoms, and the sheets
themselves are bound quite loosely. Thus any kind
of pressure will cause the sheets to fry apart,
which is why graphite, unlike diamond, is so soft.
The reason diamond is so hard has to do mainly
with its crystal structure, which describes how
the atoms pack. Diamond is made of carbon atoms
that are packed in a very specific fashion. The
carbon atoms are all closely interacting with each
other, and there are strong chemical bonds (called
covalent bonds) between the carbons. These
covalent bonds create an infinite 3-dimensional
network that is very rigid, and this is what gives
diamond its special properties.
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