The structure of crystals. Interatomic forces in crystals
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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).

Covalent bond in diamondDiamond structure showing the covalent bonds that hols atoms together
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.

Metallic bonding
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

NaCl, a ionic structure
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...

Crystal in a snowflake
Water molecules linked by H-bonds, keep the tridimensional structure of snowflakes

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