Write the general formula for alkenes.

Hint :Unsaturated hydrocarbons are those hydrocarbons (carbon-hydrogen compounds) which possess double or triple covalent bonds among its neighboring carbon atoms. Due to the occurrence of double bonds in alkene compounds, alkenes are classified as unsaturated hydrocarbons.

Complete Step By Step Answer:
To define alkenes they are unsaturated hydrocarbons with at least one carbon-to-carbon double bond. Olefins are an alternate term we use for alkenes. Because we find a double bond in alkenes, they tend to exhibit more reactive property compared to alkanes.
To give a general formula to any compound we must take some examples of such compounds and find the common pattern present in all such compounds.
Some examples of alkenes look like this:
(Each carbon should satisfy the condition of having four hydrogen atoms each)
 $ C{H_2} = C{H_2} $ , $ C{H_3} - CH = C{H_2} $ , $ C{H_3} - CH = CH - C{H_3} $
We see that when there are two carbon atoms there are four hydrogen atoms present, but if there are three carbon atoms then there are six hydrogen atoms also for four carbons there are eight hydrogens.
After careful analysis we see that for each carbon atom, twice as many hydrogen atoms will be present. So the formula should contain double the number of hydrogen atoms as compared to carbon atoms.
So the general formula can be written in this way:
For $ n $ given carbon atoms, there will be alkenes with formula $ \Rightarrow {C_n}{H_{2n}} $
Therefore the general formula is $ {C_n}{H_{2n}} $ .

Note :
The physical properties of alkenes are altered by this carbon-carbon double bond. Alkenes occur in all three states at normal temperatures: namely; liquid, solid or gases. Alkenes have equal melting and boiling points as alkanes or very similar to them; although, cis-alkenes (that are isomers) compared to trans-alkene isomers have lower melting point.