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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Like other people have said, it’s the electrons that determine almost all of an atom’s behavior that you would consider “chemistry”. That’s because the electrons exist far outside the nucleus and can interact with the electrons of other atoms.
Probably the reason we classify elements based on protons instead is because the number of protons in an atom is stable (except for the radioactive ones). Electrons can be added or removed if you have enough energy. If it’s very hot, like in stars, the electrons might pay very little attention to the nucleus, but the nucleus still has a certain number of protons and can still be identified from afar as being from a particular element.
The number of protons determine how many electrons an atom has when it’s neutrally charged, which is how the atom is happiest on its own. The other significant factor is how close that number is to the nearest full shell of electrons, either by adding or removing electrons. For example, if an atom has a full shell already, it will be very unreactive. If it’s one away, it will be very reactive. If it’s two away, it will be moderately reactive.
The electric charge that the atom has in the “full shell” state is also important, because molecules want to be electrically neutral too. So a chlorine ion that’s -1 with a full shell will be very eager to join up with a sodium ion that’s +1 with a full shell.
So we care about the number of protons, because that number tells you not only how many electrons there normally are, but also the different number of electrons the atom will have when it’s in a molecule, which determines what types of compounds it can form.
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