What is the structure for 3-ethyl-4-methyloctane?
What you are referring to here is a chain of carbon atoms, of which the base chain is eight carbons long. We know this because the last part of the name says "octane", the prefix oct- referring to the number eight. Of those eight carbon atoms, normally there would be three hydrogen atoms on carbons 1 and 8, while the interior carbons would have only two hydrogens. What has happened with this compound, however, is an ethyl group was substituted for one of the hydrogens on carbon atom 3, and a methyl group was substituted for one of the hydrogen atoms on carbon atom 4. The structure should look something like this:
H H H CH3HH H H
H C-C-C-C-C-C-C-C H
H H CH2HH H H H
You will have to imagine the vertical lines connecting the hydrogens, ethyl group, and methyl group. Compounds such as this are referred to organic compounds, because they only have the elements carbon and hydrogen in them. The branch of chemistry devoted to the study of such molecules is called organic chemistry.