Structural Formula for Cyclohexanone (and molecular formula)

Cyclohexanone is a ring with six Carbon atoms and an Oxygen atom double bonded to on of the Carbon atoms.

We can draw the structure for Cyclohexane (below) and then add the double bonded Oxygen atom.  But we need to be sure that each Carbon atom only has four bonds.  That gives us the structure for Cyclohexanone (a ketone).

----Drawing the Structure for Cyclohexane----

Cycloalkanes are made up of only Carbon and Hydrogen atoms all singled bonded, just like straight-chained alkanes. But since cycloalkanes are in a ring they have a slightly different formula (essentially two less H atoms in the molecule).

The general formula for Cycloalkanes is:
CnH2n
The lowercase n represents the number of Carbon atoms in the cycloalkane.  So for example, if you have cyclopropane, which has 5 Carbon atoms the formula would be C5H(2*5) or C5H10.  

The general formula of CnH2n applies to other cycloalkanes as well as cyclohexane.