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Structural formula - Sometimes it is useful to depict the structure of a molecule. This is commonly used in organic chemistry, the study of living (or any carbon-based) substances. There are several ways to achieve this, depend on the level of sophistication required. For example, water can be represented as follows:
 
The diagram on the left is the symbolic structural formula of water. The lines represent chemical bonds. Some molecules have 2 bonds joined between atoms. In this case, a double parallel line will be drawn. The diagram on the right show a graphical depiction of the molecule. In this case, the 'ball and stick' model is used. In this model all atoms (balls) are joined by a series of 'sticks'. Usually, only 'a stick'
is shown joining between two atoms. It does not indicate the number of bonds as in the case of symbolic structural formula.
Sometimes it is useful to illustrate a molecule in 3-dimensions. A simple example like methane (CH4) with a carbon atom at the center and four hydrogen atoms extended evenly to space.
 
Diagram on the left above show the extensions using symbolic structural formula. The solid lines indicate two C-H bonds lie on the plane of the monitor screen; the dash line show a C-H bond extends in a direction away from the viewer and solid triangular notation represent a C-H bond that extends in a direction towards the viewer.
Diagram on the right above show the corresponding 'ball and stick' model of the molecule. Finally, another form of visualization, namely the space-filled model, represents a molecule in terms of the size (approximate) of atoms in a molecule. It gives the viewer a rough guide to the actual overall size and structure of the molecule. An example of a space-filled model of methane is shown below:
Notice that the hydrogen atoms are shown as cyan spheres with part of the spheres penetrate into the carbon atom represented by the grey sphere. The overall model also indicates methane molecule to be rather spherical, which is not as clearly indicated with the other form of model representations.
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