I understand that when the quark theory was being developed that SU(3) was used to explain the mesons that were ultimately found to be composed of the up, down, and strange quarks. I also get that the SU(3) is grouped as an octet and a singlet, with the eta prime meson being the singlet. But I'm a little fuzzy on the why of some of them, since the book I'm reading doesn't exactly make it clear. That is, why is the SU(3) grouping that of an octet and a singlet? What differentiates the octet from the singlet? Why is the eta prime meson the singlet and not one of the other mesons?