In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, homology is the appearance of homologues. A homologue (also spelled as homolog) is a
compound belonging to a series of compounds differing from each other by a repeating unit, such as a
methylene bridge −−, a
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
residue, etc.
A homolog is a special case of an
analog. Examples are
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...
s and compounds with
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
side chains of different length (the repeating unit being a
methylene group
In organic chemistry, a methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by two single bonds. The group may be represented as , where th ...
-CH
2-).
Periodic table
On the
periodic table
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
, homologous elements share many
electrochemical properties and appear in the same
group (column) of the table. For example, all
noble gases are colorless,
monatomic gases with very low reactivity. These similarities are due to similar structure in their
outer shells of
valence electrons.
Mendeleev used the prefix
eka- for an unknown element below a known one in the same group.
See also
*
Homologous series
In organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series can be branched or unbranched, or differ by molecular formula of and mole ...
*
Analog
*
Congener
*
Structure–activity relationship The structure–activity relationship (SAR) is the relationship between the chemical structure of a molecule and its biological activity. This idea was first presented by Crum-Brown and Fraser in 1865.
The analysis of SAR enables the determination o ...
References
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Chemical nomenclature