How do you know when an isotope is unstable?

for my chemistry homework, we have a problem that says explan why lanthanum-127 s unstable. but the only thing i've learned that makes an isotope unstable is if the ratio of neutrons to protons is 1.5 : 1 or greater ... or i think smaller than 1:1 (im not sure about that though) and neither of those work for lanthanum-127. so, i'm wondering if there are other factors that weigh in to making an isotope unstable? Please help my teacher like didn't teach anything about this and the textbook chapter for nuclear is like 50 pages long ahh. thanks!!!