molecule

The Columbia Encyclopedia, 6th ed.

molecule

molecule (mŏl´əkyōōl) [New Lat.,=little mass], smallest particle of a compound that has all the chemical properties of that compound. A single atom is usually not referred to as a molecule, and ionic compounds such as common salt are not made up of molecules. Unlike ions, molecules carry no electrical charge.

Nature of Molecules

Molecules are made up of two or more atoms, either of the same element or of two or more different elements, joined by one or more covalent chemical bonds. According to the kinetic-molecular theory, the molecules of a substance are in constant motion. The state (solid, liquid, or gaseous) in which matter appears depends on the speed and separation of the molecules in the matter. Substances differ according to the structure and composition of their molecules. A molecular compound is represented by its molecular formula; for example, water is represented by the formula H2O. A more complex structural formula is sometimes used to show the arrangement of atoms in the molecule.

Molecules differ in size and molecular weight as well as in structure. In a chemical reaction between molecular substances, the molecules are often broken apart into atoms or radicals that recombine to form other molecules, i.e., other substances. In other cases two or more molecules will combine to form a single larger molecule, or a large molecule will be broken up into several smaller molecules.

Molecules can assume many shapes and sizes. Molecules of hydrogen gas, H2, are very small; each consists of two atoms of hydrogen. Water molecules, H2O, are much larger, containing an atom of oxygen as well as two of hydrogen. The atoms in a water molecule are arranged at the corners of an isosceles triangle; the oxygen atom is located where the two equal sides meet and the angle between these sides is about 105°. A carbon dioxide molecule, CO2, is linear, with the two oxygen atoms an equal distance on either side of the carbon atom. In methane, CH4, the hydrogen atoms are arranged at the corners of a tetrahedron with the carbon atom in the center. In benzene, C6H6, the carbon atoms form a hexagonal ring with a hydrogen atom joined to each carbon atom. More complex molecules resemble rings, chains, helices, or other forms. Many molecules occurring in living organisms are very complex. RNA and DNA molecules resemble giant helices. By polymerization a large number of small molecules may be joined to form a single large polymer molecule. Typical polymers include synthetic resins, rubbers, and plastics.

Evolution of Molecular Theory

The terms atom and molecule were used interchangeably until the early 19th cent. Initial experimental work with gases led to what is essentially the modern distinction. J. A. C. Charles and R. Boyle had shown that all gases exhibit the same relationship between a change in temperature or pressure and the corresponding change in volume. J. L. Gay-Lussac had shown that gases always combine in simple whole-number volume proportions and had rediscovered the earlier findings of Charles, which had not been published.

Dalton's Theory

One early theorist was John Dalton, best known for his atomic theory. Dalton believed that gases were made up of tiny particles, which he thought were atoms. He thought that these atoms were stationary and in contact with one another and that heat was a material substance, called caloric, that was contained in shells around the atom (these shells of caloric were actually what was in contact). When a gas was heated, the amount of caloric was increased, the shells became larger, and the gas expanded. Dalton did not accept Gay-Lussac's findings about combining volumes of gases, perhaps because it could not be explained by his theory.

Avogadro's Hypothesis

A different theory from Dalton's that could explain the combining volumes of gases was proposed by the Italian physicist Amadeo Avogadro in 1811. According to his theory, under given conditions of temperature and pressure, a given volume of any gas contains a definite number of particles. From the earlier observation that one volume of hydrogen gas and one volume of chlorine gas react to form two volumes of hydrogen chloride gas he deduced that the particles in gaseous hydrogen or chlorine could not be single atoms, but must be some combination of atoms. He called this combination a molecule. He reasoned that the two volumes of hydrogen chloride that are formed must contain twice as many particles as either single volume of hydrogen or chlorine. Thus, if there were 100 particles each of hydrogen and chlorine, there would be 200 particles of hydrogen chloride produced; but there could be only 100 particles produced if the original particles of hydrogen and chlorine were indivisible atoms, since each particle of hydrogen chloride contains both hydrogen and chlorine. An assumption that there are two atoms in a molecule of gaseous hydrogen or chlorine and one atom each of hydrogen and chlorine in a molecule of hydrogen chloride preserves both the hypothesis of indivisible atoms and the hypothesis of equal numbers of particles in equal volumes of gases. Similar reasoning would allow a larger even number of atoms in the molecules of hydrogen or chlorine, but Avogadro favored a rule of simplicity, using the smallest possible number. In the model of gases proposed by Avogadro, the particles were not in contact and much of the volume of the gas was empty space.

Cannizaro's Compromise

Avogadro's theory was not well accepted; most responses were very critical. Meanwhile, Dalton's theory prompted extensive experimentation and especially the determination of combining weights of the elements. Many shortcomings of Dalton's theory were uncovered, and although a number of modifications were suggested, none were very successful. It was not until 1858 that the Italian chemist Stanislao Cannizaro suggested a merging of Avogadro's and Dalton's theories. The acceptance of this revised theory was assisted by the acceptance by physicists at about the same time of the kinetic-molecular theory of gases that was first proposed in 1738 by Daniel Bernoulli.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

Items saved from this article

This article has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this article

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited article

molecule
Settings

Settings

Typeface
Text size Smaller Larger Reset View mode
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Help
Full screen

matching results for page

    Questia reader help

    How to highlight and cite specific passages

    1. Click or tap the first word you want to select.
    2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
    3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

    OK, got it!

    Cited passage

    Style
    Citations are available only to our active members.
    Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

    1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

    Cited passage

    Thanks for trying Questia!

    Please continue trying out our research tools, but please note, full functionality is available only to our active members.

    Your work will be lost once you leave this Web page.

    Buy instant access to save your work.

    Already a member? Log in now.

    Oops!

    An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.