Okay, so I understand that the number of protons in an atom defines the element. But why are there only 118 of them? Can’t we keep on adding protons to an atom to create new elements?

Question

EDIT: Thanks for all the responses!

In: 3337

Answer 1

The protons and neutrons in the the nucleus of an atom are held together by something that’s kind of like magnetism, called the Strong Nuclear Force. The thing is that in the same way magnets only appear to work within a certain distance, the distance the Strong Nuclear Force works is only about the width of a proton. So while it’s very, very strong, it’s only holding each proton to the ones that are “touching” it. It doesn’t hold the whole nucleus together; it just holds adjacent protons together. And because the range of the Strong Nuclear Force is so short, it doesn’t get any stronger if you add more protons.

Meanwhile, there’s another force called Electromagnetism that creates an effect called Electrostatic Repulsion. Electrostatic Repulsion causes the protons to push away from each other. However, Electrostatic Repulsion isn’t very strong when there are only a few protons. But the more protons you add, Electrostatic Repulsion grows stronger and stronger. At a certain number of protons, Electrostatic Repulsion becomes stronger than the Strong Nuclear Force.

That number of protons is 83. Any atomic nucleus with 83 or more protons is unstable because at that point, Electrostatic Repulsion is working harder to break the nucleus apart than the Strong Nuclear Force can work to keep it together. So Lead, with an atomic number of 82, is the last stable element, while Bismuth, at 83, is the first element that’s unstable. In the case of Bismuth, it would take an unthinkable amount of time for its nucleus to decay, because it’s not *very* unstable. As you go up the periodic table to elements with more and more protons, they (more or less) get more unstable as you go up.

So, TL;DR: At 83 protons in the nucleus, Electrostatic Repulsion gets stronger than the Strong Nuclear Force, so any more protons than that an the nucleus will eventually break apart.

*[Edit]: As some very correct people have pointed out, this explanation is a gross oversimplification that ignores what’s really going on in the underlying particles and forces mentioned here. I was just trying to get as close to an ELI5 answer as I could think of without going into the non-layperson parts of the explanation. But in reality, those “details” really ARE the story, and any simplification of them kind of points you in the wrong direction if you are actually trying to study the real inner workings of the atom.*