What is the difference between isobutane and n-butane ?

I want to show you a simple mnemonic to identify iso alkanes.

If you look at structural formulas of several iso alkanes what you will find is that all of them have a Y-shaped end . Just have a look at them:

You can easily see the red Y-s in their structural formulas.

So, when you see an isoalkane, think Ysoalkane . And, of course, I'm delibarately misspelling the word in order to create a good link inside my head. This is how mnemonic devices work!

Note: the official IUPAC-names would be methyl-propane, 2-methyl-butane and 2-methyl-pentane respectively, but the iso-names are still 'tolerated'.