It is a radioactive isotope that is more stable than lower proton count atoms.
One thing to consider is the odd-odd rule - odd number of protons and odd number of neutrons, especially once your reach element 81 thallium. An odd number of protons and neutrons means a very unstable nucleus. Virtually all of the nuclides of the elements from thallium and beyond are radioactive, i.e. unstable.
John, what you should do is take some time to research the strong force. Its name indicates that it is the strongest of the four known natural forces. If we arbitrarily assign gravity a force of 1 then the electromagnetic force is 10^38 and the strong force is 137 * 10^38. The strong force overcomes the Coloumb repulsion of the protons (+ charge) in the nucleus. But even through it is 137 times greater than the EM force there's a very important difference. The EM force is carried by photons...at the speed of light and, at least intheory, extends to infinity. The strong force is carried by Pi mesons. They are very massive particles. While the force is 137 times greater it only extends to the diameter of one or two nucleon (protons and neutrons) diameters. THat's the fight in the nucleus. All those protons are repelling each other with a force that extends to infinity (dropping of under the inverse square to the change in distance rule) while the strong force only extands to a couple of nucleon diameters. The nucleus of every element beyong hydrogen does its best to blow itself apart.
As you add more protons you need an ever increasing number, by increasing proportion, of neutrons to glue it together because the strong force is attractive and not affected by the EM force. But it is a losing battle. The strong force can't continue to overcome the repulsive force of EM. The nucleus is radioactive.
You might ask me just how strong the repulsion between protons is. I'd answer thusly. Stand on a platform 1 meter above the ground. Step off. You'll hit the ground at about 5 meters a second velocity. Now increase the force of gravity to equal the force of EM. In that case you'd hit the ground at about 1.6 million meters a second! Because you're a real person made of real "stuff" what would actually happen is your feet would start moving at the velocity "long" before your head moved. You'd be "spaghettified" - and then go splat.
There's a reason why we don't see elements beyond Uranium in the natural world. All of the other elements are so unstable that they have long ago decayed, assuming that a super nova actually created any ponderous mass of them in the first place.