Structurally, chlorophyll a and b are different as
Chl a has a methyl group and Chl b has an aldehyde group.
Chl a has a carboxyl group and Chl b has an aldehyde group.
Chl a has an aldehyde group and Chl b has a methyl group.
Chl a has an ethyl group and Chl b has an aldehyde group.
Chl a has a methyl group (-CH3) and Chl b has an aldehyde group (-CHO) in its structure.