I would like to explain chemical formulas is not representable in terms of the molecular structure of the compound.
Any good suggestions of examples?
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1What about carbon? Diamond, graphite, graphene, nanotubes, fullerenes... – tschoppi Aug 13 '14 at 10:41
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The classical example is dimethyl ester $\ce{CH3-O-CH3}$ and etanol $\ce{CH3-CH2-OH}$ pair. They have exactly same brutto-formula, but different structure and very different properties. The first is a gas, the second is a liquid easily reacting with sodium metal.
Carbon is also interesting example. It has two common forms with exactly same chemical formula $\ce{C}$, but very different properties: a very hard and transparent diamond and soft black graphite.
permeakra
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Examples from geology abound, and are probably more interesting to everyday folks.
Titanium dioxide, $\ce{TiO2}$ is commonly found in a rutile form that has a body-centered cubic tetragonal strucutre, but anatase has tetragonal pyramidal crystal structure. Rutile has very unusual optical properties that anatase does not have; and conversely, anatase enjoys catalytic properties that rutile does not. This system very well studied by chemists and materials scientists.
Hydrated magnesium silicate, $\ce{Mg3Si2O5(OH)4}$ comes in a bewildering array of polymorphs. Several (chrysotile, tremolite, anthophyllite, amosite, crocidolite, and actinolite) are collectively known as asbestos. Their unusual fibrous crystal structure led to their use in a variety of fire-resistant applications until it was realized that inhaling fine shards of these minerals was a major health hazard. In contrast, lizardite is also a hydrated magnesium silicate with formula $\ce{Mg3Si2O5(OH)4}$, but does not have a fibrous crystal structure and is not hazardous.
Some examples from biochemistry are also very interesting because they matter to folks' everyday lives.
- Trans fats like elaidic acid, $\ce{C18H34O2}$ were previously abundant in the food supplies of many Western countries because of partial hydrogenation of plant-derived vegetable oils. Dietary consumption of trans fats is now believed to be a major health hazard. However, the cis isomer of elaidic acid is called oleic acid. It is the major component of olive oil and other plant-derived oils, and is not believed to be hazardous. It's formula is also $\ce{C18H34O2}$.
