Molecule definition and examples

A molecule can be defined as a group of two or more atoms joined together by covalent bonds, thus forming discrete electrically neutral units of constant composition that make up the smallest possible units that maintain the composition and properties of a pure chemical substance. .

Molecules can be composed of the same class of atoms, thus forming homonuclear species corresponding to the different allotropes of the chemical elements. For example, gaseous oxygen is an allotrope of oxygen made up of O ₂ molecules (that is, molecules of two oxygen atoms), while ozone (O ₃ ) molecules are made up of three oxygen atoms and represent another natural allotrope. of said element.

Molecules can also be made up of atoms of more than one element, giving rise to molecular chemical compounds. The simplest example is water, which is made up of H ₂ O molecules, made up of two hydrogen atoms bonded to one oxygen atom.

Molecules can range from very small, made up of just two atoms (the smallest of all is the hydrogen molecule, H ₂ ) to very large, made up of thousands of atoms (such as DNA, proteins, and polysaccharides).

Molecules and ionic compounds

Although we usually represent ionic compounds such as salts and some oxides with formulas similar to those we use for molecules, it is very important to clarify that ionic compounds are not generally considered to be molecules . This is because a crystal of an ionic compound such as sodium chloride (NaCl or common table salt) is not made up of discrete units in which each ion (for example, a sodium cation) is bonded to a single counterion ( for example, a chloride anion) as its formula would seem to suggest.

In ionic bonding, oppositely charged ions are held together by electrostatic attraction, so a single cation (positively charged ion) equally attracts all the oppositely charged anions around it. It is like saying that each cation is simultaneously bound to all the anions around it in the crystalline structure and, at the same time, each anion is bound to all the cations around it.

For this reason, an ionic crystal could actually be seen as an aggregate of millions of ions, all linked together by ionic bonds, rather than as discrete, separate particles made up of groups of atoms linked together.

Why do many people refer to salts as molecules?

The confusion may come from the very common mistake of referring to the formulas with which we represent ionic compounds (NaCl, KBr, NaNO ₃ , for example) as molecular formulas, when in reality they are empirical formulas.

The same can be said for molecular weights. Only molecular compounds and elements (those made up of molecules) have molecular weight. It is incorrect to refer to the weight of NaCl as the molecular weight of sodium chloride (since NaCl is not a molecule). Instead, it should be called the formula weight, since it is nothing more than the added weight of the atoms in the empirical formula.

Molecules and solids of covalent network

Finally, it should also be clarified that not all substances in which the atoms are linked by means of covalent bonds are considered molecules. An example of this is covalent solids (or covalent network solids). These substances are made up of two-dimensional networks (sheets) or three-dimensional atoms linked together by covalent bonds.

The same thing happens with covalent lattice solids as with ionic solids: there is no discrete unit (other than individual atoms) that can represent a molecule. For this reason, substances such as graphite and diamond, which consist of networks of carbon atoms linked together, are not considered molecular substances.

types of molecules

Molecules can be classified in several ways depending on their composition, their size, and their origin:

Types of molecules according to their composition

• Homonuclear molecules: they are those that are formed by a single type of atoms or, what is the same, by atoms of the same element. These are the molecules from which molecular elements such as molecular hydrogen, oxygen, and nitrogen are made.
• Heteronuclear molecules: they are the most numerous and are formed by the union of two or more types of atoms. This means that they represent the discrete units of molecular compounds. Examples include water (H ₂ O), carbon dioxide (CO ₂ ), methane (CH ₄ ), among others.

Types of molecules according to their size

• Diatomic molecules: are those formed by only two atoms, whether they are the same or different from each other.
• Triatomic molecules: are those formed by three atoms, whether they are the same or different from each other, for example, ozone, water and carbon dioxide.
• Small polyatomic molecules: are made up of more than 3 atoms. There is no sharp border between small and large molecules, but most simple molecules such as glucose (C ₆ H ₁₂ O ₆ ), methane (CH ₄ ), and even larger complexes of up to hundreds of atoms, are still considered to be molecules. little.
• Large molecules or macromolecules: The term macromolecule is almost always reserved to refer to molecules that contain thousands of atoms, and that are also formed by the union of multiple smaller molecules called monomers, one after another. Such is the case of both natural and synthetic polymers. Examples include DNA, RNA, and the polypeptide chains of proteins.

Types of molecules according to their origin

• Organic molecules: these are those carbon-based molecules that can also contain hydrogen, oxygen, nitrogen, phosphorus, halogens, and sulfur, among others, and that come from living beings, such as hydrocarbons, alcohols, and aromatic compounds.
• Inorganic Molecules: They are the opposite of organic molecules. They originate in the atmosphere, in the soil and in bodies of water, but without the intervention of living beings.
• Biologically important molecules: Within organic molecules, there is a group of molecules that are particularly important for life and cellular processes. These molecules include water, carbohydrates, amino acids and proteins, nucleic acids, and lipids, among others.

Types of molecules according to their polarity

• Polar molecules: are those that have a net dipole moment, that is, they have polar bonds whose dipole moments do not cancel each other.
• Nonpolar Molecules: These are molecules that either have no polar bonds, or the dipole moments of all their bonds cancel each other due to molecular symmetry.

Examples of large and small molecules

Ozone (O ₃ )

Ozone is an example of an inorganic homonuclear, triatomic molecule.

The bucky ball or fullerene (C ₆₀ )

_{C 60} fullerene is one of the molecular allotropes of carbon. It is made up of molecules of 60 carbon atoms that form a sphere similar to a soccer ball, so it is a homonuclear molecule.

Dinitrogen tetroxide (N ₂ O ₄ )

This brown gas is made up of N ₂ O ₄ molecules , which are small heteronuclear inorganic molecules.

Acetone ((CH ₃ ) ₂ C=O)

Acetone is an example of a polar organic molecule.

Sucrose (C ₁₂ H ₂₂ O ₁₁ )

This carbohydrate is a disaccharide (formed by two sugar molecules linked together) and represents an organic molecule of biological importance.

Globular Actin (G Actin)

Globular actin is an example of a protein, that is, a biological macromolecule, in this case formed by the union of 374 amino acids that form a globular structure.

References

Brown, T. (2021). Chemistry: The Central Science. (11th ed.). London, England: Pearson Education.

Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.

Flowers, P., Neth, EJ, Robinson, WR, Theopold, K., & Langley, R. (2019). Chemistry: Atoms First 2e . Retrieved from https://openstax.org/books/chemistry-atoms-first-2e/pages/1-introduction