Lecture Notes from CHM 1341
11 June 1996

ATOMS are the brain child of Democritus and Lucretius (see "De Rerum Natura," Titus Lucretius Carus' 400 BC text-in Latin- "On the Nature of Things" for the place of atoms in the philosophy of the ancient world) and Dalton (early 1800s). Democritus argued the existence of individual, irreducible units of matter from philosophical grounds. Dalton argued their existence from the constant combining proportions of many elements.

Their size is small (0.00000002 cm) but not inconceivably small; consider that a 1 cm line of atoms could be cut in half by some infinitely sharp knife only 27 times before trying to divide one of its members.

While neither Democritus nor Dalton could see atoms, we have been able to do so since the work of Dr. Albert Crewe at The University of Chicago in the late 1970s. Today, STM (Scanning Tunneling Microscopy) is so routine that even U.T. Dallas has two groups using it: Dr. Inga Musselman in Chemistry and Dr. Grover Wetsel in Microelectronics. Dr. Musselman uses a variant, Atomic Force Microscopy, to characterize the surface of solid samples, and Dr. Wetsel uses the scanning probe to push and pull atoms on a surface in efforts to manufacture truly microelectronics. Their work and that of others means we need no longer ask General Chemistry students to take atoms and molecules on faith!

Individual and collective atoms of the same kind are called "elements," making them the modern equivalent of "earth, water, fire, and wind," the elements of the ancients. They are symbolized in the Periodic Table with 1 or 2 letters each drawn from their current (e.g., Mg for MaGnesium) or ancient (e.g., Pb for "PlumBum," the Latin for "lead"; Romans were good plumbers) names. Their compounds, intimately-bonded rather than loosely-mixed aggregations, use these symbols along with their combining proportions in their molecules, such as:
Fe₂O₃
implying that a single molecule of iron (III) oxide, "rust," contains 2 atoms of iron (Fe from the ancients' "ferrum") and 3 of oxygen. Were one to decompose rust and discover that 2 to 3 proportion, one could report it's "empirical formula" as given above, but unless one also knew that its molecular weight was consistent with the formula as written, the proportions alone would not distinguish between the following two forms:

Fe₂O₃ or Fe₄O₆
or indeed any other multiple. An egregious example is given by almost all the carbohydrates, such as glucose (your literal brain food):

C₆H₁₂O₆, the sugar glucose
whose "empirical formula," determined by proportions alone would give the C:H:O ratios always to be 1:2:1. Only the additional information that the molecular weight of glucose is 6 times that of

CH₂O
would indict the 6:12:6 version as the actual "molcular formula."

MOLECULES of different types in collection may be mixtures if they cluster like-with-like in the mix, but they also may intermix perfectly all the way down to the molecular level as do water and ethanol in spirits. If the molecules do dissolve in one another, they may do so as solids or gases in addition to liquids. Metallic tooth fillings are examples such solid solutions; they are known as amalgams. And all gases will dissolve in one another (unless they undergo spontaneous chemical reaction, which is rare). But there are plenty of examples of liquids which are immiscible; there's the old saw, "Oil and water don't mix!" Well, almost true; they certainly do not dissolve in one another to form a homogeneous solution, but one can homogenize oily milkfat in water to make a suspension which appears to be a solution; still it's actually a mixture of tiny fat droplets in water.

That means that, in common with all mixtures, different small regions of milk will have very different properties. It is this inhomogeneity (even in "homogenized" milk) which is a key property of mixtures. SOLUTIONS, on the other hand, are mixed at the molecular level; so no subregion can be found with properties different from any other.

MIXTURES also imply the pollution of one "pure" compound with another. If the major component is in high proportion, it makes sense to speak of its purity in terms of the impurity level, such as 1 ppm ("one-part-per-million"), another way of saying 99.99999% ("six 9s") pure. Often it is the case that the exact magnitude of an small impurity is of less significance than its destructive effect on the intended use of the "pure" major component. And there are extreme cases where even a single molecule is of critical significance; the male moth antennae can respond to a single molecule of the female moth's pheremone!

SIGNIFICANT FIGURES are only those to which the uncertainties in your calculations entitle you to. As a rule of thumb, if, in your Chemistry calculations, values have differing uncertainties, the solution should reflect the minimum significant figures of any component of the calculation. Zeroes listed to the LEFT of non-zero values have no "significance;" they are mere placeholders which establish the magnitude of the number. But zeroes appearing to the RIGHT of non-zero numbers add to the number of significant digits. All this is to caution you that your hand calculator hasn't a clue how FEW digits are meaningful in your calculations; you must decide how many to believe at the end!

Chris Parr
University of Texas at Dallas
Programs in Chemistry, Room BE3.506
P.O. Box 830688  M/S BE2.6 (for snailmail)
Richardson, TX  75083-0688


   Voice: (214) 883-2485
     Fax: (214) 883-2925
     BBS: (214) 883-2168 (HST) or -2932 (V.32bis)
Internet: parr@utdallas.edu  (sends Chris e-mail.)