Natural radioactive nuclei are mostly nuclei of high mass number why ?

Compare the radii of the nuclei of mass numbers 27 and 64.

In a sample of a radioactive substance what fraction of the initial number of nuclei will remain undecayed after a time t =T/2 where T=half-life of radioactive substance.

A particular nucleus in a large population of identical radioactive nuclei did survive 5 halt lives of that isotope. Then the probability that this surviving nucleus will service the next half life is

What happens to half life of radioactive nuclei at very high temperatures?