How do I explain differences in surface area for these alkanes?
(A) Neopentane vs n-pentane
(B) Isobutane vs butane
(C) Isobutane vs n-propane

Hint: Alkanes are the saturated hydrocarbons, consisting of only carbon and hydrogen atoms. The alkanes that occupy more volume have more surface area. The alkanes with more surface area have more boiling points also. Branched alkanes have a volume like sphere which is less than straight chain alkanes.

Complete answer:
The general molecular formula of alkanes is \[{C_n}{H_{2n + 2}}\] , where n is the number of carbon atoms. Alkanes can exist as straight chain and branched chain alkanes.
Longer is the carbon chain, more will be the volume, and more will be the surface area.

(A) Neopentane vs n-pentane:
Neo-pentane is a five membered branched alkane, in which the four methyl groups were attached to the same carbon only. It has a volume like a sphere. N-pentane has five carbon atoms and is longer than isopentane. Thus, neopentane has more surface volume than n-pentane.

(B) Isobutane vs butane
Isobutane is a branched alkane consisting of a volume-like sphere, whereas butane is a straight chain alkane. Thus, isobutane has more surface area than butane.

(C) Isobutane vs n-propane
Isobutane has four carbon atoms, and propane has three carbon atoms. Isobutane has more volume than propane. Thus, isobutane has more surface area than propane.

Note:
Chemical compounds are classified into functional groups based on the groups present in it. Alkanes, alkenes, and alkynes are the hydrocarbons. Alkanes are the saturated hydrocarbons consisting of only carbon and hydrogen atoms. More will be surface area, higher is the boiling point. Thus, the boiling point is also evidence of more surface area.