3.4.11 Structure Determination - Data sources
Students should:
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Structure
Pure substances may be either elements or compounds. The term structure' means the actual 3-dimensional arrangement of atoms, molecules or ions within a chemical element or compound. Pure substances can have four possible structural types:
- 1 Ionic
- 2 Simple covalent
- 3 Giant covalent
- 4 Metallic
The type of structure is important when choosing the most suitable method of analysis.
Determination of structure
Determination of structure (discovering the structure of a compound) is a very important part of chemistry. The problem is that the molecules, or crystal lattices, cannot be 'seen' as such and indirect evidence for the structure must be gleaned by a variety of methods. This is particularly important in organic chemistry where the structure may be complicated.
A structural determination may be derived from evidence gathered from a variety of techniques, each one providing evidence leading towards the final conclusion. It is not likely that one technique alone provides sufficient evidence.
The final 'proof' is often the synthesis of the compound from simpler units, producing a substance with identical characteristics to the unknown.
Determination of purity
Pure substances have sharp, well-defined melting temperatures. Their boiling points are also specific, but are a function of the atmospheric pressure and consequently dependent on the conditions. Melting points are lowered and become less well-defined (sharp) by the presence of impurity.
However, although the melting point is affected by impurities, the trace impurities often encountered in analysis may not cause any appreciable lowering of the melting point.