Atoms and Compounds

Chm 1311 Lecture for 7 June 2000

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are Not
by Chemical

Composition puts things together from their components. So decomposition must separate them into their components again.

Chemical decomposition is no different. However, there comes a time when chemical decomposition is of no effect; that is when the substance is already in its most elementary components. For chemical substance, those primary components are called The Elements. We go the recent movie (5th Element) 107 better; while it described the ancient "elements" of earth, water, wind, and fire (and added a specious but shapely 5th), the chemical elements number 112...last year. It is unlikely that that's all there'll be by the turn of the century.
As an example, if we heat chalk, calcium carbonate or CaCO3 , to a high enough temperature, it will decompose into lime, CaO, and carbon dioxide, CO2. The chemist would write that as:

CaCO3 + D  arrow right CaO + CO2

This means that CaCO3 was a compound (of elements) since it was decomposed. But neither of those products are elements since they can be further decomposed; lime, for example, can be electrolyzed as a melt (even higher heat) as follows:

2 CaO + D  arrow right 2 Ca + O2

where now neither of the products can be decomposed further. They must both be elements. So lime is a compound which can be decomposed into its elements, calcium and oxygen. (By the way, although we chemists use D in some places like the mathematicians do, to mean a difference, here it is used to symbolize heat as the old alchemists would have used it.)
Of course, we can go another way as well. We can mix elements or even compounds together without chemical means so that they remain the substances they were but are now in a mixture. Clearly a mixture of pure substances is not a pure substance itself. And it can be unmixed, not by chemical means (which changes compounds into others) but rather by physical means.

of Mixtures

Many are already well known to you. As you evaporate a solution of salt water, the water (a pure compound H2O) separates from the solute (a pure solid NaCl). But while we have there the advantage of separate physical phases, it's certainly possible to separate pure liquids by boiling of the component of lower boiling point via distillation.
Chemists exploit other differences in property beside boiling point. A difference in freezing points means one should be able to cool and precipitate out the higher melting compound. More subtle such differences include the dwelling time of a compound on some solid support within a fluid stream. The component less attracted to that support is eluted by the stream earlier than the others in a process called chromatography. (A drop of colored ink on a paper towel shows differential migration of its colored components...the "chroma" in chromatography.)

Subscripts vs.

The "3" in CaCO3 is a formula subscript which simply saves us from having to write CaCOOO. That could get tedious! So the subscripts show the ratio of elements (shown as atomic symbols: Ca C and O) in the compound. Those subscripts imply that there is a fixed ratio of elemental atoms in a chemical compound.

The non-subscripted numbers in 2 CaO  arrow right 2 Ca + O2 specify how many substance molecules result from that decomposition; 2 CaO molecules go into the reaction but 3 molecules come out. These coefficient numbers are called Stoichiometric (from the Greek: stoichio "element" metric "measure"). No chemical reaction is complete unless the stoichiometric coefficients and the formula subscripts show that there are the same number of all atoms (however bound) on both sides of the reaction arrow. When there are, the chemical equation is said to be "balanced."
It's a properly balanced chemical equation which permits the chemist to know minimum amounts of possibly precious materials (s)he can use to produce a required amount of some useful product. But it was the fixed nature of the elemental ratios as specified by the formula subscripts that first suggested to scientists that elements existed as atomic or molecular units.

John Dalton

John Dalton, 1766-1844

MS Encarta '97
John Dalton, an unassuming English schoolmaster, had read his Lucretius, the Roman poet of De Rerum Natura (On the Nature of Things) in which are expounded (among other things) the notions of the Greek, Democritus, who spoke of "atomos" in the void. That captured the essence (another Greek philosophical word) of gases, but what of solids which would appear to have no "void" in them? Listen to Lucretius (30 AD):

  • And again,
    What seems to us the hardened and condensed
    Must be of atoms among themselves more hooked,
    Be held compacted deep within, as 'twere
    By branch-like atoms - of which sort the chief
    Are diamond stones, despisers of all blows, . . .
And Dalton found the evidence of those "hooks". He reasoned that if sulfur formed two oxides wherein 32 g of S would combine with either 32 g of O or 48 g of O, then the ratio of O atoms in those two sulfur oxides must be 2:3! Fixed ratios of atoms make for fixed compounds.
And he was right. Those two compounds are indeed SO2 and SO3. Dalton had correctly inferred the existence of atoms and their combination into molecules. And his insight merely rested upon the natural assumption that all atoms of the same element weighed exactly the same. Or rather he thought that a necessary condition, but it's not. All that's required is that in any macroscopic amount of a compound, any differing elemental weights always appear in the same abundance.


And they do. And it's called the Natural Abundance of isotopes of elements. However, if we're going to compare the weights of atoms, the SI mass unit, kg, is a little too large. Nowadays we use 1/12 of the weight of the most abundant "isotope" of the carbon atom as the atomic weight unit, u. The 1/12 is so the lightest atom, hydrogen, comes out nearly 1 u, but carbon's easier to work with.
Then, for example, about 50% of bromine atoms weigh 79 u (the 79Br isotope) and the other 50% weigh 81 u (the 81Br isotope). Since the percentage never varies, the average bromine weighs 80 u even though no individual atom of bromine does! Doesn't matter. With the enormous number of atoms involved in any weighable amount, the average bromine weight is always found to be the same since the % abundances never vary.

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Last modified 25 May 2001. Chris Parr