Chemical Compositions

Mixture or "pure substance" ?

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

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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 S8 molecule 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 Science magazine; 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.