A molecule is the smallest part of a substance that determines its properties. It is composed of two or more atoms bonded together.
The simplest molecule is hydrogen gas, where two hydrogen atoms come together while sharing their electrons. There are much more complex molecules, with hundreds and even thousands of atoms linked together, as is the case of proteins, starch and other polymers.
There are monatomic molecules, that is, they are made up of a single type of atom, such as hydrogen gas H2, molecular oxygen with two oxygen atoms O2 and ozone with three oxygen atoms O3.
Other molecules combine different atoms, such as water, where two hydrogen atoms bond to one oxygen atom, and carbon dioxide, where one carbon bonds to two oxygens.
Examples of different molecules and their chemical formulas.
Characteristics of the molecules
Molecular mass: The mass of a molecule is the sum of the masses of the atoms that make it up. For example, the molecular mass of water is 18 atomic mass units (amu), which is equal to the mass of two hydrogens (1 amu) and one oxygen (16 amu).
Molecular structure: Molecules can have a linear shape, like carbon dioxide, a triangular shape like the water molecule, or forming rings, like benzene.
Molecular size: Molecules can be diatomic, with two atoms in their composition, such as carbon monoxide CO and hydrochloric acid HCl; triatomics such as carbon dioxide CO2 and water H2O; polyatomic like glucose which has 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms C6H12O6.
Polarity: Molecules can be polar or non-polar. The polarity of molecules is given by their structure and the difference that their atoms have in attracting electrons from other atoms. Thus, water is a polar molecule, because oxygen attracts the electrons of the hydrogens, concentrating electrons at one of the points of the molecular triangle.
Intermolecular forces: Attraction forces are established between molecules that allow their union.
See also: Atom, Chemical element, Ion, Covalent bond, Macromolecule, Biomolecules, Levels of organization of matter
References
Atkins, P. Jones, L., Laverman, L. (2016) Chemical principles-The Quest for Insights.
Salvat, J. (director) (1987) Salvat Encyclopedia of Science and Technology. Salvat Editors. Barcelona.
Zumdahl, S.S., Zumdahl, S.A. (2014) Chemistry 9th ed. Cengage Learning. AC.