In science it is necessary to know exactly what we are talking
about, so before we can even begin to consider matter from a chemical
point of view, we need to know something about its composition; is the stuff I am looking at a single substance, or is it a mixture?
(We will get into the details of the definitions elsewhere, but for the
moment you probably already have a fair understanding of the
distinction; think of a sample of crystalline salt (sodium chloride) as
opposed to a solution of salt in water— a mixture of salt and water.)
To a chemist, there is a fundamental distinction between a pure substance and a mixture.
But marketers, and through them, the general public, don't
hesitate to describe a complex mixture such as peanut butter as "pure".
Pure what?
Elements and compounds
It
has been known for at least a thousand years that some substances can
be broken down by heating or chemical treatment into "simpler" ones, but
there is always a limit; we eventually get substances known as elements
that cannot be reduced to any simpler forms by ordinary chemical or
physical means. What is our criterion for "simpler"? The most observable
(and therefore macroscopic) property is the weight.
The idea of a minimal unit of chemical identity that we call an element
developed from experimental observations of the relative weights of
substances involved in chemical reactions. For example, the compound
mercuric oxide can be broken down by heating into two other substances:
2 HgO → 2 Hg + O2
... but the two products, metallic mercury and dioxygen, cannot be decomposed into simpler substances, so they must be elements. The definition of an element given above is an operational
one; a certain result (or in this case, a non-result!) of a procedure
that might lead to the decomposition of a substance into lighter units
will tentatively place that substance in one of the categories, element
or compound. Because this operation is carried out on bulk matter, the
concept of the element is also a macroscopic one.
Painting by Joseph Wright of Derby (1734-97) The Alchymist in Search of the Philosopher's Stone discovers Phosphorus
Elements and atoms
The atom, by contrast, is a microscopic concept which in modern chemistry relates the unique character of every chemical element to an actual physical particle.
The idea of the atom as the smallest particle of matter had its
origins in Greek philosophy around 400 BCE but was controversial from
the start (both Plato and Aristotle maintained that matter was
infinitely divisible.) It was not until 1803 that John Dalton proposed a
rational atomic theory to explain the facts of chemical combination as
they were then known, thus being the first to employ macroscopic
evidence to illuminate the microscopic world. It
took almost until 1900 for the atomic theory to became universally
accepted. In the 1920's it became possible to measure the sizes and
masses of atoms, and in the 1970's techniques were developed that
produced images of individual atoms.
← Cobalt atom imaged by a scanning tunneling microscope
Formula and structure
The formula of a substance expresses the relative number
of atoms of each element it contains. Because the formula can be
determined by experiments on bulk matter, it is a macroscopic concept
even though it is expressed in terms of atoms.
What the ordinary chemical formula does not tell us is the order in which the component atoms are connected, whether they are grouped into discrete units (molecules)
or are two- or three dimensional extended structures, as is the case
with solids such as ordinary salt. The microscopic aspect of composition
is structure, which in its greatest detail reveals the
relative locations (in two or three dimensional space) of each atom
within the minimum collection needed to define the structure of the
substance.
Macroscopic
Microscopic
Substances are defined at the macroscopic level by their formulas or compositions, and at the microscopic level by their structures.
The elements hydrogen and oxygen combine to form a compound whose composition is expressed by the formula H2O.
The molecule of water has the structure shown here.
Chemical substances that cannot be broken down into simpler ones are known as elements. The actual physical particles of which elements are composed are atoms or molecules.
Sulfur-the-element in its orthorhombic crystalline form.
The S8molecule is an octagonal ring of sulfur atoms. The crystal shown at the left is composed of an ordered array of these molecules.
(This animation does not properly represent the actual vibrational motions of the molecule.)
Compounds and molecules
As we indicated above, a compound is a substance
containing more than one element. Since the concept of an element is
macroscopic and the distinction between elements and compounds was
recognized long before the existence of physical atoms was accepted, the
concept of a compound must also be a macroscopic one that makes no
assumptions about the nature of the ultimate .
Thus when carbon burns in the presence of oxygen, the product
carbon dioxide can be shown by (macroscopic) weight measurements to
contain both of the original elements:
C + O2 → CO2
10.0 g + 26.7 g = 36.7 g
One of the important
characteristics of a compound is that the proportions by weight of each
element in a given compound are constant. For example, no matter what
weight of carbon dioxide we have, the percentage of carbon it contains
is (10.0 / 36.7) = 0.27, or 27%.
Molecules
A molecule is an assembly of atoms having a fixed composition, structure, and distinctive, measurable properties.
In its most general meaning, the term molecule can describe any
kind of particle (even a single atom) having a unique chemical
identity. Even at the end of the 19th century, when compounds and their
formulas had long been in use, some prominent chemists doubted that
molecules (or atoms) were any more than a convenient model.
Computer model of the nicotine molecule,
C10H14N2, by Ronald Perry ↑
Molecules suddenly became real in 1905, when
Albert Einstein showed that Brownian motion, the irregular microscopic
movements of tiny pollen grains floating in water, could be directly
attributed to collisions with molecule-sized particles.
Finally, we get to see one! In 2009, IBM scientists in Switzerland succeeded in imaging a real molecule, using a technique known as atomic force microscopy
in which an atoms-thin metallic probe is drawn ever-so-slightly above
the surface of an immobilized pentacene molecule cooled to nearly
absolute zero. In order to improve the image quality, a molecule of
carbon monoxide was placed on the end of the probe.
The image produced by the AFM probe is
shown at the very bottom. What is actually being imaged is the surface
of the electron clouds of the molecule, which consists of six hexagonal
rings of carbon atoms with hydrogens on its periphery. The tiny bumps
that correspond to these hydrogen atom attest to the remarkable
resolution of this experiment.
The original article was publshed in Sciencemagazine; see here for an understandable account of this historic work.
The atomic composition of a molecule is given by its formula. Thus the formulas CO, CH4, and O2
represent the molecules carbon monoxide, methane, and dioxygen.
However, the fact that we can write a formula for a compound does not
imply the existence of molecules having that composition. Gases and most
liquids consist of molecules, but many solids exist as extended
lattices of atoms or ions (electrically charged atoms or molecules.) For
example, there is no such thing as a "molecule" of ordinary salt, NaCl
(see below.)
Confused about the distinction between molecules and compounds?
Maybe the following will help:
A molecule but not a compound - Ozone, O3, is not a compound because it contains only a single element.
This well-known molecule is a compound because it contains more than one element. [link]
Ordinary solid salt is a compound but not a molecule. It is built from interpenetrating lattices of sodium and chloride ions that extend indefinitely.
