Question

where does the n come from in a compound such as n-butane?

Answer 1

The n stands for "normal" meaning that the chain is a straight chain and not branched. I can't draw structures on the board but here is a site that will illustrate the difference between a straight chain and an isomer of that compound. Here is a site that shows a number of compounds but look at n-butane (a straight chain compound) and isobutane (a branched chain). They have the same formula of C4H10 but one has four carbons in a row and the other has only three carbon atoms in a row with the fourth one attached to the middle carbon.
http://www.geo.arizona.edu/xtal/nats101/9_16.jpg