Definition of molecule in chemistry: a molecule is an electrically neutral entity composed of two or more atoms of the same element or different elements, joined by a covalent chemical bond.
In organic and biochemical chemistry, the term molecule sometimes also identifies polyatomic ions, while in the kinetic theory of gases it is often used for each gaseous particle, regardless of its composition: with this definition, even the individual atoms of the family of noble gases can be considered molecules.
A molecule can be composed of multiple atoms of a single chemical element or atoms of different elements. The families of molecules formed by the same atoms arranged in space differently are called isomers, and the arrangement affects the physical properties of the substance.
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 called in one direction the so-called infinite chains, if they extend in two in one directions we call them infinite chains), if they extend in three directions we call them infinite three-dimensional structures. In the latter cases the molecules are called giant molecules. As examples of giant molecules we have an ionic crystal, a molten salt, a solid or molten metal, a covalent solid (such as diamond or silica,
The solid molecular fingers (consisting of crystals where the molecules occupy the nodes of the reticulum) and the molecular fluids, normal or associated, are aggregates of discrete molecules. In these types of structures the forces that maintain cohesion between the 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 clearly higher than intramolecular ones.
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, discrete molecules can only be macromolecules in the condensed state when they are constituted by a very large number of atoms. Discrete macromolecules can also be grouped in an orderly fashion forming crystals.
In the case of gases, the molecules are animated at room temperature with rapid movements of translation and rotation. In the case of molecular solids and liquids, there are mostly oscillations around equilibrium positions.
The atoms that make up the molecules are in geometrically defined positions although there are vibrating movements inside the molecules. In this way discrete molecules are shaped and you can talk about distances and link angles. However, these distances and these angles are not absolutely fixed.
The kinetic energy (rotational, vibrational) and the electronic energy of the 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 become such that the molecules dissociate into the atoms that compose them.
Although the molecules are electrically neutral, the molecules are made up of atoms (electrons, protons and neutrons), that is, charged particles. We call nonpolar 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 identified chemicals. For this reason it is important to define the characteristic parameters of the molecules such 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. This is the octet rule, which is not always fulfilled.
Molecular model of valence bond
The most updated valence bond model considers the overlap (axial or lateral) of atomic orbitals of different atoms. This theory is complemented by the introduction of orbital hybridization, which allows the spatial configurations of molecules to be explained.
Model of molecular orbitals
A third model is that of molecular orbitals. According to this model, the atomic orbitals link 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.
Currently, molecular research is increasingly focusing on complex molecules (for example biological molecules) and macromolecules (for example polymers).