An isomer (/ˈaɪsəmər/; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same chemical formula as another molecule, but with a different chemical structure. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like positional isomers, cis-trans isomers and enantiomers, etc. (see chart below). There are two main forms of isomerism (/ˈaɪsəmərɪzm/ or /aɪˈsɒmərɪzm/): structural isomerism and stereoisomerism (spatial isomerism).
In structural isomers, sometimes referred to as constitutional isomers, the atoms and functional groups are joined together in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group. This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism, which deals with the position of a functional group on a chain; and functional group isomerism, in which one functional group is split up into different ones.
Isomer (Greek isos = "equal", méros = "part") is an element of transverse body articulation of the bilateral fossil animals of the Phylum Proarticulata from the Ediacaran (Vendian) period. This term has been proposed by Andrey Yu. Ivantsov, a Russian paleontologist from the Laboratory of the Precambrian organisms, Paleontological Institute, Russian Academy of Sciences.
Proarticulatan isomers are distinct from the segments of the Annelida and Panarthropoda, as each of these elements occupies only half of width of a body and are organized in an alternating pattern relatively to the axis of the body. In other words, although proarticulatans are bilaterally symmetrical, one side is not the direct mirror image of its opposite. Opposite isomers of left and right side are located with displacement of half of its width. This phenomenon is described as the symmetry of gliding reflection.
The first element in the row is a right isomer on the anterior end of the dorsal side. All successive isomers are similar to it, but gradually decrease in size and in angle of inclination from the anterior part (head) of the body to the posterior end. New isomers were added at the growth point at the posterior end of the proarticulate body. With age, the addition of new isomers slowed down and probably stopped, the growth point shifted from the posterior end, and the relative length of the posterior isomers increased, sometimes significantly. The overall number of body isomers amounts from several pairs (Vendia, Onega) to several hundreds (Dickinsonia).