molecule, a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance. caffeine moleculeEncyclopædia Britannica, Inc. The division of a sample of a substance into progressively smaller parts produces no change in either its composition or its chemical properties until parts consisting of single molecules are reached. Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties. In this latter stage of fragmentation the chemical bonds that hold the atoms together in the molecule are broken. water moleculeEncyclopædia Britannica, Inc. Atoms consist of a single nucleus with a positive charge surrounded by a cloud of negatively charged electrons. When atoms approach one another closely, the electron clouds interact with each other and with the nuclei. If this interaction is such that the total energy of the system is lowered, then the atoms bond together to form a molecule. Thus, from a structural point of view, a molecule consists of an aggregation of atoms held together by valence forces. Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical, as in, for example, the oxygen molecule (O2), they compose a homonuclear diatomic molecule, while if the atoms are different, as in the carbon monoxide molecule (CO), they make up a heteronuclear diatomic molecule. Molecules containing more than two atoms are termed polyatomic molecules, e.g., carbon dioxide (CO2) and water (H2O). Polymer molecules may contain many thousands of component atoms. The ratio of the numbers of atoms that can be bonded together to form molecules is fixed; for example, every water molecule contains two atoms of hydrogen and one atom of oxygen. It is this feature that distinguishes chemical compounds from solutions and other mechanical mixtures. Thus hydrogen and oxygen may be present in any arbitrary proportions in mechanical mixtures but when sparked will combine only in definite proportions to form the chemical compound water (H2O). It is possible for the same kinds of atoms to combine in different but definite proportions to form different molecules; for example, two atoms of hydrogen will chemically bond with one atom of oxygen to yield a water molecule, whereas two atoms of hydrogen can chemically bond with two atoms of oxygen to form a molecule of hydrogen peroxide (H2O2). Furthermore, it is possible for atoms to bond together in identical proportions to form different molecules. Such molecules are called isomers and differ only in the arrangement of the atoms within the molecules. For example, ethyl alcohol (CH3CH2OH) and methyl ether (CH3OCH3) both contain one, two, and six atoms of oxygen, carbon, and hydrogen, respectively, but these atoms are bonded in different ways.
Ins and Outs of Chemistry You may know that elements make up the air we breathe and the water we drink, but do you know more about them? What element is almost as light as hydrogen? What do you call a blend of two chemical elements? Find out the answers in this quiz. Not all substances are made up of distinct molecular units. Sodium chloride (common table salt), for example, consists of sodium ions and chlorine ions arranged in a lattice so that each sodium ion is surrounded by six equidistant chlorine ions and each chlorine ion is surrounded by six equidistant sodium ions. The forces acting between any sodium and any adjacent chlorine ion are equal. Hence, no distinct aggregate identifiable as a molecule of sodium chloride exists. Consequently, in sodium chloride and in all solids of similar type, the concept of the chemical molecule has no significance. Therefore, the formula for such a compound is given as the simplest ratio of the atoms, called a formula unit—in the case of sodium chloride, NaCl.
There are 34,000 different species of fish. That’s more than all the other vertebrates (amphibians, birds, reptiles, and mammals) combined. See All Good FactsMolecules are held together by shared electron pairs, or covalent bonds. Such bonds are directional, meaning that the atoms adopt specific positions relative to one another so as to maximize the bond strengths. As a result, each molecule has a definite, fairly rigid structure, or spatial distribution of its atoms. Structural chemistry is concerned with valence, which determines how atoms combine in definite ratios and how this is related to the bond directions and bond lengths. The properties of molecules correlate with their structures; for example, the water molecule is bent structurally and therefore has a dipole moment, whereas the carbon dioxide molecule is linear and has no dipole moment. The elucidation of the manner in which atoms are reorganized in the course of chemical reactions is important. In some molecules the structure may not be rigid; for example, in ethane (H3CCH3) there is virtually free rotation about the carbon-carbon single bond. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now
Updated March 31, 2020 By Chris Deziel Reviewed by: Lana Bandoim, B.S.
Since John Dalton established the existence of atoms in the early 1800s, they have been considered the building blocks of matter. Scientists now know that atoms themselves are built from smaller particles, which are in turn composed of even smaller ones, and nobody really knows how far the regression goes.
On the constructive side, however, atoms combine to form all the chemical compounds that comprise the physical world and everything in it.
Fill in the blank: "A __ is composed of two or more atoms held together by chemical bonds." There is more than one answer, but the one that probably crossed your mind first is "molecule." Every grouping of two or more atoms is a molecule. Some are very simple, such as the oxygen molecules you breathe, which are formed by two oxygen atoms (O2), and some are huge, like the TTN genes in the human body. Composed of carbon, hydrogen, oxygen, nitrogen and sulfur, the so-called Titan molecule is composed of a whopping 539,030 atoms.
The other word you can use to fill in the blank is "compound." A compound is a molecule that is a substance made of two or more elements chemically combined in a set ratio. It contains more than one element, or type of atom. A compound is always a molecule, but a molecule isn't always a compound. The Titan molecule is a compound, and simpler examples of compounds include include sodium chloride (NaCl), or table salt, and dihydrogen oxide (H2O), or water.
To understand how atoms combine, remember that they are composed of smaller particles. They are called electrons, protons and neutrons. Electrons have a negative charge, protons have an equal positive charge, and neutrons have no charge. An atom has an equal number of electrons and protons, which makes it electrostatically neutral, but if electrostatic neutrality was all that mattered, atoms would never combine.
Electrons circle the nucleus in discrete orbits, or shells, that can hold a fixed number of them, and that number increases with the radius of the orbit.
If an atom is missing electrons to fill a shell, usually its outer one, it is unbalanced, and to gain stability, it seeks to get them from another atom in one of two ways. It can "steal' an electron (more generously, the other atom can "donate" it), or the two atoms can share electrons. Either way, the atoms become bonded chemically to form a molecule.
When an atom donates an electron to the other, both atoms become ions, each with an opposite charge, and they are bonded by electrostatic attraction. This is called an ionic bond. When atoms share electrons to complete each other's outer shells, they form a covalent bond, which isn't as strong as an ionic bond, but it's much more common.
If you don't like to call molecules compounds, you can distinguish them as homonuclear, which means made of one element, or heteronuclear, which means made of more than one element. H2, O2 and P4 are examples of the former, while CO2, HCl and CH4 are examples of the latter. The list of heteronuclear molecules is obviously much longer than that of homonuclear ones, as most molecules are compounds.
Molecules aren't always electrostatically neutral. Ionic molecules combine in a way that leaves them with a charge, and they can form ionic bonds with other molecules. Some molecules, such as the water molecule, are polar, because the way the atoms combine creates a net positive charge on one side and a negative charge on the other. This charge isn't as strong as the one that forms a chemical bond, but it's strong enough to produce some strange and important behavior. |
A type of molecules, made up of one or more types of atom, held together by chemical bonds.
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