Elements like sulfur and chlorine or gold and mercury seem really different in their chemical properties but are right next to each other on the periodic table.
From what I remember from high school, it’s not the nucleus but the outer electron shells which define chemical properties.
Chemical bonding capabilities are depending on the willingness to accept a shared electron or the ability to lent a shared electron, in salts like Na+ and Cl-. The breaking of chemical bonds, like creating H2 and O2 from water, are also happening in the electron shells.
The more to the right of the period table, the fuller the outer electron shells and the harder to make it react with something.
Protons actually have little influence over the way atoms interact with one another. Electrons, on the other hand, are the subatomic particle capable of coming into contact (so to speak) with other atoms.
Electrons live in different orbits around the nucleus. The inner orbits don’t interact much with other atoms, so we can ignore them and focus instead on the outer orbits. The number of electrons in the outermost orbit determines how an element will interact with other elements. Will it lend it’s extra electrons out to another atom? Will it borrow electrons from somewhere else? Is it happy being by itself? All this is determined by the electrons in the outer orbit.
The periodic table is designed so that elements who behave similarly are grouped together. They are not, however, grouped by being next to one another. Instead, they are grouped in columns.
For example, all of the elements in the left-most column of the periodic table have one element in their outer shell. They behave similarly when introduced to other elements and compounds. They all react explosively when introduced to water.
The elements in the far right column, on the other hand, have full outer electron shells. They are largely non-reactive, and they are called the nobel gasses.
There are other ways to group elements, but that’s the basics.
If you are collecting the 151 original Pokemon cards, and you only have 30 or 50, it’s no big deal. But if you only need 1 or 2 cards to complete your collection, you are a little more dangerous. You are more likely to spend a lot of money on packs, to buy a card outright, or even steal a card outright. But once you have a full collection, you are less likely to care if someone wants to trade cards.
If you are a group of a handful of protons, and you are missing one electron to complete your valence shell, you are way more likely to want to cut throats to get that last electron. If you have a full collection, you wouldn’t care about anyone or anything because you are already complete.
Ultimately, gold medalists are happy, bronze medalists are happy, and silver medalists are miserable.
An extra proton changes two things: the amount of electrons and the space they occupy. Simply put, when you add a proton you must add an electron to match. This electron can’t be where any other electrons already are. When other atoms are around a given element the electrons of the two atoms talk. Where the electrons are ultimately dictates how they talk. That is to say, electron distribution determines bonding strength and angles, which determine chemical properties. Remember, chemical properties are electrical properties.