Isomers

Isomers are compounds with the same molecular formulae but different structural formulae.

They have the same number of each type of atom. They can be part of the same or different homologous series, e.g. alkenes and cycloalkanes can be isomers.

Due to their different structures their physical properties (melting point, boiling point etc) are usually different.

This diagram shows butane and methylpropane. (Methylpropane used to be called iso-butane, but the correct name is methylpropane).

Butane has the molecular formula C4H10. Its structural formula is a carbon atom bonded to three hydrogen atoms, joined to a carbon atom with 2 hydrogens. This is joined to another carbon with two hydrogens, which is joined to a carbon with 3 hydrogens. Methylpropane also has the molecular formula C4H10. Its structural formula is a carbon atom bonded to three hydrogen atoms, joined to a carbon atom with one hydrogen and a methyl group. This carbon is joined to another carbon with three hydrogens.

Both have a molecular formula of {C_4}{H_{10}}. The structural formulae are different though, so they are isomers of each other.

Isomers are molecules with the same formula but different chemical structures.

Polar and ionic compounds tend to be soluble in polar solvents, whereas non-polar compounds tend to be soluble in non-polar solvents. By looking for polar bonds and arrangement of bonds in a solvent, solubility can be predicted.

Boiling point for compounds tends to increase with molecular size due to increasing intermolecular forces. When comparing molecules of the same size, the type of intermolecular forces present need to be considered by looking for O-H or N-H bonds (hydrogen bonding) or polar bonds (permanent dipole-permanent dipole interactions).

Volatility is a measure of the ease of evaporation. Again this is affected by the intermolecular forces present.