Question

205 Tl is expected to be stable. True or False?

the answer is true, but I don't understand why if the ratio of N/Z is 124/81?

Answer 1

Only for lighter nucleus (up to Z of 20), A/Z ratio should be 1 to be stable as the nuclear force of attraction counteracts the repulsive forces between the positive charged protons.

However as the size of the nucleus increases the number of neutrons need to increase and thus A/Z ratio increases and becomes more than 1. It is because as the size of nucleus increases the repulsive forces among more numbers of protons increases and cannot be counteracted by nuclear forces of attraction. So by increasing number of the neutral neutrons the interaction among protons reduces, leading to increased stability.