An alkane with molecular formula C5H12 can exist in the following isomeric forms :
(I)

H3CCH2CH2CH2CH3

n−pentane

(II)

H3C CH3 | C HCH2CH3

2−methylbutane

(III)

H3C− CH3 | C | CH3 −CH3

2,2,dimethylpropane

Upon photochemical chlorination, the H atom in the alkane is replaced by halogen atom. Looking at the structures we find that 3 different halogen derivatives are possible for (I), while 4 are possible for (II) and only I for (III).
(i) All the H atoms in (III) are equivalent. Therefore, we get only a single monochloride from (III).
(ii) (I) yields 3 isomeric products due to presence of 3 different H atoms.
(iii) (II) yields 4 isomeric products due to presence of 4 different H atoms.