Question

How many monochloro isomers of 2-methylpropane are there?

Answer 1

Only two

Explanation:

If you look at the alkane, only the C-2 is different from the rest. You may see it as a `CH-`group with three equivalent `CH_3-`groups attached.
So the single chloro may attach to the "central" C, or to one of the others.

1-chloro 2-methyl propane
2-chloro 2-methyl propane

Since the methyl group may only be attached to the C-2 of the propane main chain, the -2- may be left out:

1-chloro methyl propane
2-chloro methyl propane

If the chloro is on the 2-methyl group, then we renumber, so this becomes the C-1, and the original C-1 is now the methyl carbon.