Re: What is the difference between an ethyl and a methyl?

According to IUPAC rules, you need to name a compound based on the longest 
LINEAR chain possible. 
For example if you have CH3-CH2-C(CH3)H-C(CH3)H-CH3, although there are 7 
carbons in all, the longest straight line chain consists of 5 carbons. The 
other two CH3 groups are branches. The basic name thus becomes pentane, 
not heptane. To then indicate the presence of the other two groups 
(CH3=methyl), we use the full name 2,3-dimethyl-pentane. The "2" and"3" 
indicate that the second and third carbon atoms within the pentane chain 
are where the branches are located, the di- indicates that there are two 
methyl groups. ( We use the lowest #'s possible to indicate position; so 
it would be incorrrect to use 3 and 4 if you started to count from the 
left hand side)
If instead of CH3, you had the following chains, you would then use the 
corresponding names that I've included:

CH3CH2----ethyl
CH3CH2CH2---propyl
CH3CH2CH2CH2--butyl

For more details, clearer structural formulas, and more examples, consult 
the organic chemistry text Organic Chemistry by Solomon or
Organic Chemistry by Boyd or email me with specific examples at 
enricouva@hotmail.com