Here's why:
* Strong Nuclear Force: The strong nuclear force holds protons and neutrons together in the nucleus. This force is very strong at short distances, but it weakens rapidly with distance.
* Electromagnetic Force: Protons, being positively charged, repel each other due to the electromagnetic force.
* Neutron-to-Proton Ratio: Neutrons provide the "glue" that helps overcome the electromagnetic repulsion between protons. As the number of protons increases, the electromagnetic repulsion becomes stronger, requiring more neutrons to maintain stability.
Key Points:
* Magic Numbers: Nuclei with specific numbers of protons and neutrons, known as "magic numbers" (2, 8, 20, 28, 50, 82, and 126), are particularly stable due to closed shells of protons and neutrons, similar to electron shells in atoms.
* Beyond Iron: Iron (Fe) is the most stable element in the periodic table. Elements heavier than iron tend to be less stable because they have more protons, leading to stronger electromagnetic repulsion. They can achieve stability by undergoing radioactive decay.
In summary: While a neutron-to-proton ratio close to 1.5 is a general indicator of stability in heavy nuclei, the interplay of the strong and electromagnetic forces, along with the concept of magic numbers, plays a significant role in determining the relative stability of different isotopes.
