We explain what molecules are, how they are classified and their properties. Also, what are its characteristics and examples.
What are Molecules
A molecule is called an electrically neutral, structured and interrelated set of atoms that belong to the same chemical element or to several. For example, the dihydrogen (H 2 ) molecule is made up of only hydrogen atoms, while the methane (CH 4 ) molecule is made up of one carbon and four hydrogen atoms .
These sets are formed thanks to interatomic attractive forces called chemical bonds , due to which structures composed of atoms are formed that are more stable than separate atoms.
Another way that has been used to define a molecule but is sometimes not accepted by some scientists is that it is the smallest part, which still retains its physical and chemical properties , into which a chemical substance can be divided. According to this definition, there are molecules made up of a single atom, such as helium (He).
The properties of a specific substance are determined by the properties of the molecules that make it up. Therefore, the structure of the molecules of a substance and the strength of the interactions between them will depend on whether it is a solid (little separation between molecules), a liquid (medium separation) or a gas (a lot of separation ).
There are other chemical compounds that are also made up of electrically neutral sets of atoms, but they are not molecules. Among them are the minerals that make up the Earth , ionic ( salts ) or covalent crystals and other similar materials (such as glass or metals ). In other words, one cannot speak as such of a molecule of glass, or iron , or aluminum .
How were the molecules discovered?
The idea that matter is composed of indivisible particles bound together is as old as Ancient Greece , when the philosopher Democritus proposed it.
However, the atomic hypothesis in scientific terms was only proposed in 1803 , when the English chemist John Dalton verified that matter would be formed from stable bonds of atoms called molecules.
The nature of atoms and of these molecules was explored in greater depth by later scientists such as Thompson and Rutherford, who laid the foundations for contemporary chemistry.
The definition of a molecule changed as technology advanced and a more detailed study of the atomic and molecular structure became possible.
Types of molecules
Macromolecules are chains made up of simpler molecules.
Molecules can be classified according to various criteria. According to the size and complexity of their structure, they are classified into:
- Discrete molecules . They have a defined number of atoms, either of different elements or of the same, and can be classified in turn according to the number of atoms in their structure. Thus we have monatomic (one single atom), diatomic (two atoms), triatomic (three atoms), tetraatomic (four atoms), etc. For example, dinitrogen (N 2 ) is a diatomic molecule.
- Macromolecules and polymers . They are large molecular chains, made up of repeats of simpler molecules, linked together to achieve extensive and complex sequences, with new and specific properties. These macromolecules are key to the development of the chemistry of life . For example, DNA and proteins .
It is important to clarify that all polymers are macromolecules, but not all macromolecules are polymers. Polymers are macromolecules made up of simpler molecular units called monomers, which are repeated throughout their structure. On the other hand, macromolecules are large molecules that may or may not have a repeating monomeric unit.
According to their polarity (separation of charges in the molecular structure), they are classified into:
- Polar molecules . They are made up of atoms that have different electronegativity. The electrons move towards the most electronegative atom, generating a negative charge density on it and a positive charge on the other atom. This separation of charges on the molecule results in the formation of an electric dipole, that is, the molecule is left with a certain electric charge. For example, water (H 2 O).
- Nonpolar molecules . Its atoms have an identical electronegativity, without inequalities with respect to the attraction of electrons by atomic nuclei. These molecules do not have an electrical charge associated with the uneven distribution of their electrons. For example, molecular oxygen (O 2 ).
According to the predominant chemical element in its main structure, they are classified as:
- Organic molecules . Its main structure is based on carbon atoms. They are essential for life. For example, methane (CH 4 ) and amino acids (constituents of proteins).
- Inorganic molecules . Its main structure is not based on carbon. It is important to clarify that there are also inorganic molecules with carbon atoms in their structure, but this is not their main element. For example, carbon monoxide (CO) and ammonia (NH 3 ).
Properties of molecules
The reactivity of molecules is determined by their atoms and their three-dimensional structure.
The main properties of molecules are:
- Polarity . It is the separation of electrical charges on the molecular structure, because electrons are more attracted to the more electronegative atoms. This property is essential to understand the solubility of substances, since polar substances dissolve in polar substances, while nonpolar substances dissolve in nonpolar substances, although there are always midpoints of polarities.
- Melting temperature . Temperature at which a solid turns into a liquid. This property depends on the size of the molecule.
- Boiling temperature . Temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, then the liquid turns into gas. This property depends on the size of the molecule, but also on its geometry and its interactions with other molecules.
- Volume . Molecules occupy a certain volume in space.
- Chemical reactivity . Molecules are more or less reactive depending on the atoms that form them, the geometry of their structure and the external conditions in which they are found.
- Isomerism . Two molecules have isomerism if they have the same amount of each type of atom, but in each of them these atoms are arranged differently. That is, they have different chemical structures despite having the same number and type of atoms.
Hydrogen bonding is an especially strong intermolecular force.
Intermolecular forces are interactions that occur between molecules. They are generally based on their polarities and their geometry. These forces can be of various types:
Van der Waals forces :
- Permanent dipole - permanent dipole . They occur due to the electromagnetic attraction between two polar molecules.
- Instantaneous dipole - induced dipole . Called “London forces”, they are weaker and take place among all chemical substances, when their electronic cloud is temporarily unbalanced towards one end of the molecule, it makes it possible to attract or reject the electronic clouds of other nearby molecules, and transform them by induction in a momentary dipole. They usually occur between nonpolar molecules.
- Dipole - induced dipole . They occur between a polar molecule and a nonpolar one. The charge density of the polar molecule distorts the charge density of the nonpolar molecule, inducing a dipole.
Hydrogen bridge bonding :
Some scientists call this type of bond hydrogen bridging, a definition that is sometimes considered wrong. But regardless of how it is defined, it is the electrostatic interaction between an electronegative atom from one molecule and a hydrogen atom from the other.
Differences between ions and molecules
Molecules and ions differ mainly in that the latter are electrically charged chemical units, while molecules are neutral .
The loss or gain of electrons from an atom causes ions to form. Ions can have a permanent positive electrical charge if the atoms lose electrons (cations) or negative if they gain electrons (anions). For example, the sodium cation (Na + ) and the chloride anion (Cl - ).
Due to this electrical charge, the way ions react is very different from how molecules do.
On the other hand, there are also polyatomic ions , which are groupings of atoms that are linked by covalent bonds , but that as a whole have an electric charge. For example, the nitrate ion (NO 3 - ).
Molecules have greater stability than their separate atoms.
The chemical stability of molecules has to do with the energy contained in their chemical bonds and with their molecular network.
The more energy the molecule has, the more reactive and less stable it is. To break chemical bonds, usually energy is consumed and form them almost always releases energy .
That is why molecules tend to have greater stability than their separate atoms . And that’s also the reason why atoms tend to stick together with each other to form molecules.
Differences between atom and molecule
Molecules are made up of atoms that are bonded together , so atoms are much smaller and simpler than molecules.
Larger molecules are made up of smaller, simpler molecules , which become parts or “clusters.”
That is why most molecules can undergo various procedures for breaking ( lysis ) of their chemical bonds, in order to obtain simpler molecules (if possible) or atoms belonging to certain chemical elements.
Examples of molecules
Some examples of common molecules are:
- Hydrogen : H2
- Hydrochloric acid : HCl
- Carbon dioxide : CO2
- Sulfuric acid : H2SO4
- Ethanol: C2H5OH
- Phosphoric acid: H3PO4
- Chloroform: CHCl3
- Glucose: C6H12O6
- Sucrose: C12H22O11
- Acetone: C3H6O
- Cellulose: C6H10O5
- Ammonia: NH3
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