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The Three-Dimensional Structure of Molecules - Isomerism

Structure Theory in Organic Chemistry

To understand the behaviour of molecules it is important to know their three-dimensional structure. For example, questions could be whether the atoms of a molecule are arranged linearly or angularly? Has the molecule a flat or spatial shape? As a result of their tiny size, molecules are not visible and their structure can only be elucidated in an indirect way. The structure of a molecule is inferred from its behavior. Firstly, it is important to know the composition of a molecule. The information about the type and the atomic ratio of elements, yielded by elementary analysis of a large number of compounds, results in the following general statement:

  • Dependent of its position in the periodic system, all elements can only have a certain number of bonds. This is called valency.
Tab.1
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Fig.1
Methane

A carbon atom can form four bonds. It is tetravalent.

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Fig.2
Water

Oxygen is divalent.

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Fig.3
Boron trifloride

Boron is trivalent.

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Fig.4
Hydrogen chloride

Hydrogen and the halogens are monovalent.

  • In addition, the atoms of several types of elements can form one or more bonds to the same atom. A carbon atom for example can form one, two, three or four bonds to another carbon atom.
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Fig.5
Ethane
Fig.6
Ethylene
Fig.7
Acetylene

Ethane has a carbon-carbon single bond, ethylene a double bond, acetylene a triple bond.

Exercise 1

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