Carbon atoms can connect to each other by forming
several kinds of chemical bonds. The different bonding schemes
among the carbon atoms can form different crystalline structures.
Three of these are described below.
Graphite:
Space Group C6/mmc C-centered hexagonal
Lattice parameters a = 2.4612 Angstroms
c = 6.7079 Angstroms
Atoms/unit cell 4
Cell volume 35.189 x 10-24cm3
X-ray density 2.2670 g/cm3
How are the carbon atoms
of graphite arranged?
Crystalline graphite consists of parallel
sheets of carbon atoms, each sheet containing hexagonal
arrays of carbon atoms. Each atom is connected to three
nearest neighbors, within the sheets, by covalent bonds
that separate them by a distance of 1.415 Angstroms. This
bonding arrangement results from the sp2 hybridization of
carbon's electronic orbitals. Another intriguing aspect
of the bonding scheme within the graphite sheets is the
distributed pi bonding between the carbon atoms. This distributed
pi bonding gives rise to delocalized electrons that makes
graphite electrically conducting. The sheets are held together
by weak Van der Waals forces and are separated from each
other by a distance of 3.35 Angstroms.
Diamond:
Space Group Fd3m face-centered cubic
Lattice parameters a = 3.5670 Angstroms
Atoms/unit cell 8
Cell volume 45.385 x 10-24cm3
X-ray density 3.5155 g/cm3
How the carbon atoms in
diamond are linked together?
The figure above shows the unit cell of a diamond
crystal. This unit cell shows the smallest group of carbon
atoms, arranged in three-dimensions that can represent the
essential features of the diamond crystal. The edges for this
cube are 3.5670 Angstroms long.
Each carbon atom in diamond is surrounded by
four nearest neighbors. They are connected together by covalent,
sigma, bonds that separate them by a distance of 1.5445 Angstroms.
The angles between these bonds are 109 degrees. As a result,
the central atom and its neighbors form a tetrahedron. The
interlocking network of covalent bonds makes the diamond structure
very rigid.
Buckminster Fullerene:
Space Group Fm3m body-centered cubic
Lattice parameters a = 14.14 Angstroms
Atoms/unit cell 240 (4 molecules)
Cell volume 2.827 x 10-21cm3
X-ray density 1.693 g/cm3
The C60 molecule, nicknamed
Buckyballs, is the roundest molecule formed in nature.
This crystalline structure is different from
the diamond or graphite crystal in that distinct molecules
form the unit cell of the crystal. The C60
molecules are arranged into a face-centered-cubic unit
cell. The sides of this cubic cell measures 14 Angstroms.
Each C60 molecule have a diameter of
10 Angstroms. The molecules are held together in the crystal
by weak Van der Waals forces.
In this space-filling model, each carbon atom
is represented by a wedge. The carbon 60 atoms are bonded
together in an array of hexagons and pentagons, like a soccer
ball. These molecules are called buckminster
fullerenes in honor of Buckminster Fuller who first
designed similarly shaped geodesic domes.
Click here to find
out how to calculate the density of a crystal using these
data.
