The most commonly noticed forces in crystals are the weak Van der waals forces. Consequently, the melting point of crystals is never very high (1, 2).
Structures of many substances, including crystals, vary with temperature. This ability of various substances to exist in more than one form is known as Polymorphism. Allotropy is a synonym for polymorphism and is usually used in the context of elements. Dimorphism is the word reserved for a substance that exists in two forms. Yet another definition exits for polymorphism. It also refers to the multiple crystals that might form owing to improper solvents used during the process of crystallization.
Structures of almost all substances consist of bonds, which can be intermolecular or intramolecular. In these, occasionally, one finds that hydrogen (H) bound to a strongly electronegative element (X) acquires a positive charge owing to the bond polarization by the electronegative element (represented as X- H+). Such a polarity charged hydrogen is available for interaction directly with the electronegative elements of adjacent molecules, and the resultant intermolecular bond is referred to as Hydrogen Bond (1,2) represented as three dots:
It is this hydrogen bond that accounts for the unusually high boiling points of some liquids, viz., Water (H2O), Hydrogen fluoride (HF), etc. Hydrogen bond is also involved in dimer formation as in carboxylic acid, and is the bond responsible for the stability observed in nucleic acids. When hydrogen bonding is present in crystals, it significantly affects the crystal molecular geometry (2). Thus, hydrogen bond profoundly influences the physical and chemical properties of various substances.
Continuing the discussion on similar lines, in a covalent bond, the electrons between atoms or groups with different electronegativities tend to be polarized towards the more electronegative constituent. In such situations, a partial charge can be attributed to the constituents owing to the partial ionic nature of the bond. The ability of an atom in a molecule to attract electrons towards itself is termed electronegativity (EN). The ionic character of the bond can be used as a measure of the magnitude of this effect (the partial charge/EN). When the effect is small, the bond is referred to as a polar bond and treated using dipole moments (DM). Covalent bonds are expected to have a DM of zero, provided the electrons are shared equally by the two atoms (1, 2).
In a way, DM is a quantitative measure of polarity, with Debye as units. If bond angles are known, DM is estimated by vector addition of individual bond moments. Possession of a dipole moment permits direct interaction with electric fields or with the electric component of radiation (1).
With measurements comes mathematics. Whenever a process/object/concept is characterized in terms of mathematics, relatively simple manipulation of variables can be achieved fundamentally to determine how the process, object or concept behaves in different situations. Such an exercise is traditionally referred to