[Nam]

I was reading through the general chemistry book and it said that the only stable nuclides with N/Z <1 are ¹H and ³He. So I just wonder if the size of these elements caused this exception or something else.

[Wastrel]

The more protons in a nucleus the greater the proportion of neutrons needs to be for it to be stable.  This may be due to nuclear charge.  It wasn't obvious that He-3 would be stable from theoretical grounds, prior to the discovery of tritium, H-3 was thought to be stable and He-3 radioactive.