Why does increasing or decreasing the number of protons (as you move about the periodic table) drastically change how elements look and behave. How is it with slightly different quantities of the same thing get you from gold to salt to helium? Are those atomic particles really are there is to matter?
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It has less to do with the protons themselves, and much more to do with the electrons. But the more protons an atom is, the more electrons it’s able to juggle at once. Combined with some weird quantum physics effects (orbital quantization, mainly) and means that, indirectly, adding more protons creates a very different situation for the electrons.
For example – how many electrons an atom can juggle at once depends on how strong the positive charge coming from the nucleus is – the bigger the positive charge, the harder it pulls of negatively charged things like electrons. But “orbital quantization” says that electrons can only orbit the nucleus at certain distances*, and each orbit** can only hold a certain number of electrons. Once you’ve added enough electrons that you can’t fit any more into the orbit, they have to go into an orbit further away. But the “opposite charges attract” force is dependent on distance, so electrons further away aren’t held on as tightly, and are more easily lost. That makes it easy for that atom to become a positive ion.
But as you keep adding more protons, and therefore more electrons, the positive charge in the nucleus keeps getting bigger, but the electrons can’t get any further out until they fill up the current orbit. So the nucleus pulls *harder* on electrons making them harder to lose – and easier to steal from other atom. That makes that atom likely to become a negative ion.
Adding more protons does do *something* though in the form of adding mass.
* (I know this isn’t technically true, wavefunction probability density blah blah blah, this is ELI5 though)
** (Yes I know the difference between shells and orbitals)
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