See Even Mass Beta Stability. and See Odd Mass Beta Stability.
There are no stable isobars of A=5 or 8, corresponding to Li-5 and He-5, or Li-8, Be-8 and B-8, because of alpha decay in these nuclides.
Note the distinctive alternate comb shape, due to the odd/even effect.
All isobars would have at least one stable member if it were not for alpha decay in some of them. Beryllium-8, which might be expected to be stable lying on the line of stability, is highly unstable, decaying with a half life of much less than 0.001 picosecond into two helium-4 nuclei. The fusion reactions within stars would proceed very much more rapidly and explosively if beryllium-8 did not have such a short existence.
The A=5 and A=8 isobars are the only isobars below A=211 for which there are no stable nuclides. Above A=209, there are very few isobars with stable members because of alpha decay, and the few that do exist are actually unstable with long halflives.