The arrangement of atoms within a molecule dictates its physical, chemical, and spectral properties. Small discrete, or large repeating arrangements of atoms which give rise to measurable changes in a molecule's reactivity , boiling point , melting point , and other characteristics are called functional groups. Given the structural formula of a molecule, a chemist can identify functional groups present (e.g. aldehyde, carboxylic acid, alcohol, etc.) and can postulate characteristic reactivity and physical properties for a given molecule based on the presence of these groups. Therefore, the identification of functional groups present within an unknown compound is a key step in qualitative organic synthesis and structure elucidation; it is routinely practiced by chemists to validate the synthesis of novel small molecules or identify unknown structures in complex mixtures. Techniques for assigning functional groups based on 'rules of thumb' or by matching profiles from known databases are commonly applied in organic chemistry , metabolomics , and forensic sciences . Furthermore, monitoring of functional group changes can be used to determine the progress of a reaction, and can even be used to identify the components of complex mixtures for a reaction coordinate.