An isomer is a different arrangment of a molecule, they are especially important in Organic Chemistry because there are so many of them.
These are compounds with the same molecular formula but different structures. This means that you have got the same atoms and bonds, but they are in different places.
The most common type of structural isomer is a chain isomer, this is where you have a methyl, ethyl etc branching off of the main chain. There are actually slight differences in the physical properties of this type of isomer. The branched isomers, have weaker Van der Waals forces and hence have a lower boiling point.
The second type of isomer you could have is a position isomer, this is where the functional group is in a different place. An easy to illustrate example is with haloalkanes.
The functional group, in this case - Cl, is in a different place, therefore the above diagram shows two different position isomers.
Structural and Geometrical Isomerism
Geometric isomerism is a type of stereoisomerism where there are different spatial arrangments of functional groups relative to the double bond. There are two types, depending on whether the functional group is on the same of opposite side of the double bond.
On the left is a cis isomer, where the two groups are on the same side of the double bond. This type of isomer usually has a slightly higher boiling point.
On the right is a trans isomer, where the two groups are on opposite sides of the double bond (think of transatlantic where you cross to opposite sides of the ocean). It will have a lower boiling point because it is less polar.
A structural isomer is one of three types of isomerism where the bonding is changed. This includes chain isomerism where branching occurs eg. methlybutane. Positional isomers, which have the functional group in a different place (DO NOT confuse this with the next type). And finally functional group isomers have the same atoms, but they are bonded in such a way to create a different functional group. For example: carbonyls.
We also have steroisomerism which is a different arrangement of molecules in 3D space. One type you have already encountered: geometrical isomerism also known as cis-trans isomerism; resulting from the lack of rotation on a C=C bond. However, there is a further type of steroisomerim known as optical isomerism.
If a carbon atom has 4 different groups bonded to it, it is said to be chiral and has an asymmetric carbon. For example, see the molecule below.
Optical isomers tend to be mirror images of each other, you may think that the molecules above are both the same, but if you construct them using a model set for example, you will see they can not be superimposed on each other. Optical isoers are known as enantiomers.
Optical isomers are practically the same molecule (some are treated differently by enzymes). Except that when plane-polarised light is passed through a solution, one enantiomer will rotate the light in one direction, and the other in the opposite direction.
However, if there are equal amounts of the two enantiomers it creates a racemic mixture or a racemate. If plane-polarised light is beamed throgh this, there will be no effect since the solution has no optical activity. Racemates form in addition reactions on planar double bonds (like C=C or C=O), since attack could occur on either side of the plane.