What is a molecule?
A molecule is a set of chemically bonded atoms. The electric charge of the molecules is neutral.
There is a definition of an older molecule that is less general and less precise: "A molecule is the smallest pair of a substance that can have independent and stable existence while retaining its chemical properties and certain physicochemical properties." By this definition there could be molecules with a single atom.
Types of molecules
Molecules can be formed in two different ways:
- Discrete molecules: molecules can be formed by a well defined number of atoms (usually a small number). The entities that constitute in this case are called discrete molecules. These molecules usually exist both in the gaseous state and in the condensed state. An example of discrete molecules are hydrogen or glucose molecules.
- Giant molecules. In this second case the molecules can be formed by aggregates of atoms or ions that exist only in the condensed state. These structures extend indefinitely in space. In this case we distinguish three sub-groups: if they are extened in one direction they are called infinite chains, if they extend in two in one direction we call them infinite chains), if they extend in three directions we call infinite three-dimensional structures. In these latter cases the molecules are called giant molecules. As examples of giant molecules we have an ion crystal, a molten salt, a solid or molten metal, a covalent solid (such as diamond or silica, in which all atoms are attached to nearest neighbors by covalent bonds of equal strength ).
The solid molecular fingers (constituted by crystals where the molecules occupy the lattice knots) and the normal or associated molecular fluids are aggregates of discrete molecules. In these types of structures the forces that maintain the cohesion between molecules (van der Waals forces or hydrogen bonds) are much weaker than the valence forces that bind the atoms inside the molecule. Consequently, intermolecular distances are markedly higher than intramolecular distances.
What are macromolecules?
Macromolecules are giant molecules where the bonds between atoms are covalent in nature.
Discrete molecules can also be macromolecules. In this case, the discrete molecules can only be macromolecules in the condensed state when they are constituted by a very high number of atoms. Discrete macromolecules can also be clustered in an orderly fashion to form crystals.
Movement of molecules
In the case of gases the molecules are animated at room temperature with rapid movements of translation and rotation. In the case of solids and molecular fluids there are mainly oscillations around equilibrium positions.
The atoms that make up the molecules are in geometrically defined positions although there are vibration movements inside the molecules. In this way the discrete molecules have shape and you can speak of distances and of angles of connection. However, these distances and angles are not absolutely fixed.
Energy of molecules
The rotational, vibrational and electronic energies of molecules can be quantified. Energy exchanges with electromagnetic radiation give rise to molecular spectra. The total energy of the molecules depends on the temperature (in particular the energy of the vibration movements of the atoms inside). At sufficiently high temperatures the vibrations are converted of such amplitude that the molecules dissociate in the atoms that compose them.
Although molecules are electrically neutral, molecules are made up of atoms (electrons, protons and neutrons), that is, charged particles. We call non-polar molecules if the distribution of electric charges is symmetric. In cancer, we call polar molecules if it is asymmetric.
Characteristics and models of molecules
The number of chemical elements discovered is 116. However, there are two million chemical substances identified. For this reason it is important to define the characteristic parameters of the molecules as the energy of their bonds, their lengths, their angles and the dielectric moment in addition to molecular geometry.
The formation of molecules can be illustrated with a series of models. The oldest is the Lewis model, which explains the formation of molecular bonds by the need to obtain for each atom an electronic configuration of noble gas. It is ttata of the octet rule, which is not always true.
Molecular model of valence bond
The valence bond model, more updated, considers the overlap (axial or lateral) of atomic orbitals of different atoms. This theory is complemented by the introduction of orbital hybridization, which allows explaining the spatial configurations of molecules.
Model of molecular orbitals
A third model is that of molecular orbitals. According to this model the atomic orbitals are linked together to form molecular orbitals. Molecular orbitals are shared by the two atoms that form the bonds. The electrons that are located in the molecular orbitals belong to the nuclei of both atoms instead.
Molecular research is now increasingly focusing on complex molecules (eg biological molecules) and on macromolecules (eg polymers).
Last review: November 8, 2016