The structure of
crystals. Interatomic
forces in crystals
Crystals are formed
by atoms, ions, and/or molecules that pack together
in ordered and periodic arrangements. Therefore, the existence
of these
crystal structures means that there exist interactions holding
atoms and molecules together. These forces can be classified into
two groups: the covalent forces, fundamentally
responsible for holding together the atoms that make up the molecules,
and the non-covalent
bond, which in turn is classified into many other types.
Covalent
bonding
Among the
interatomic forces holding atoms together, the strongest one
is represented by the covalent
bond, where
two atoms share their outer electrons to form the bond.
Covalent
bonding makes very strong connections between the atoms, so it is hard
to break these molecules or atoms apart. One of the best known examples
is the diamond structure, in which every carbon atom is covalently
linked to
other four carbon atoms (see figures below).
The single covalent bond between two
carbon atoms in diamond shares 2 electrons from their outer
shells. In diamond, each carbon shares electrons
with four other carbon atoms - forming four single bonds. This is like
a giant covalent structure - it continues on and on in three
dimensions.
Atoms
in molecular compounds,
organic and biological, are held together through strong connections:
the covalent bonds.
Non-covalent
bonding
Non-covalent
bonds are not as strong as the covalent
bonds, but
the
additive effect of many non-covalent
bonds can stabilize a molecule.
In contrast
to covalent bonds, non-covalent bonds do not share electrons.
This type of bonding includes:
Excluding the metallic
bonds, the
non-covalent interactions are the dominant type of interactions between
atoms and molecules and are critical in maintaining the
three-dimensional
structure of organic and large molecules, such as proteins and nucleic
acids, as
well as their corresponding crystal structures.
Metallic bonding
Metallic
bonding can be understood as a collective interaction of a
mobile electron fluid with metal ions (see figure on the right). This
type of bonds occurs in the
structure of metals, that is, when the
number of valence electrons is only a small fraction of the
coordination number; then neither an ionic nor a covalent bond can be
established.
The metallic bond is somewhat weaker than the ionic and covalent
bond.
The
metallic bonding in silver
Ionic bonding
Ionic
bonds are
strong electrostatic attraction forces
formed between positive and negative ions. This bond is
non-directional, meaning that the pull of the electrons does not favor
one atom over another. Ionic solids can be
composed of simple ions as seen in NaF (sodium fluoride, figure on the
right), or can be
composed of polyatomic ions as seen in ammonium nitrate NH4NO3
with
NH4+ and NO3-
ions (see
figure below).
The ionic structure of ammonium nitrate
Formation of an ionic bond in sodium
fluoride
The
ionic structure of sodium fluoride
Van der Waals forces
Van
der Waals forces
(partially known as London
forces) are the
residual attractive or repulsive forces between molecules or atomic
groups that do not arise from a covalent bond, or electrostatic
interaction of ions or of ionic groups with one another or with neutral
molecules. Like
hydrogen bonds, van der Waals forces rely on dipoles, a difference in
charge
between two molecules. But unlike hydrogen bonds the van der Waals
dipole usually is not permanent, but transient.
Van der Waals forces are
relatively weak compared to covalent bonds,
but play a fundamental role in fields as diverse as supramolecular
chemistry, structural biology, polymer science, nanotechnology, surface
science, and condensed matter physics. Van der Waals forces define many
properties of organic compounds, including their solubility in polar
and non-polar media.
Attractive forces resulting from
dipole-dipole interactions
Hydrophobic
interactions
Hydrophobic
interactions
describe the relations between water and
hydrophobes (low water-soluble molecules). Hydrophobes are nonpolar
molecules and usually have a long chain of carbons that do not interact
with water molecules. The mixing of fat and water is a good example of
this particular interaction (water and fat do not mix!). The
non-polar molecules group together to exclude water. By doing so they
minimize the surface area in contact with the polar solvent.
This type of interactions are
important factors driving protein folding
as well as the insertion of membrane proteins into the nonpolar lipid
environment. They also contribute to the stability of
protein-small molecule associations.
A
diagram showing atoms and amino acid
residues involved in hydrophobic interactions (spoked arcs)
Hydrogen bonds
A hydrogen
bond (or H-bond) results when a hydrogen atom that is
covalently bound
to an electronegative atom (e.g. O, N, S) is shared with another
electronegative atom.
The hydrogen bond is often described as an
electrostatic dipole-dipole interaction. However, it also has some
features of covalent bonding: it is directional and strong, produces
interatomic distances shorter than the sum of the van der Waals radii,
and usually involves a limited number of interaction partners.
These hydrogen-bond attractions can occur between molecules
(intermolecular) or within different parts of a single molecule
(intramolecular). They are stronger than the van der Waals
interactions, but weaker than covalent or ionic bonds. This type of
bonds can occur in inorganic molecules such as liquid or solid water, in organic molecules or
in macromolecules like DNA and proteins.
Example
of organic molecules linked
through H-bonds
Hydrogen bonds in liquid water are
constantly being made and broken. The lifetime of these single
hydrogen bond is very short (≈ 1 ps). Such broken hydrogen
bonds
will often simply reform.
H-bonds
in liquid water are continuously being made and remade
In
the solid and crystalline water, ie, in the snow crystals, the hydrogen
bonds are permanent and are the most responsible for the structure of
these crystals...
Water
molecules linked by H-bonds, keep
the tridimensional structure of snowflakes
But let's go
back...
