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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Yes, for the most part, the number of protons in an atom is all that really matters (pun not intended). The number of protons very strongly affects the number of electrons in an atom. An electron can fairly easily be ripped off (e.g. by X-rays) and atoms can also pull in extra electrons. But overall, the number of electrons in an atom is very strongly dependent on the number of protons.
The number of electrons, in turn, affects several different properties that we can observe. Electrons live in “shells” or “clouds” surrounding the nucleus. The number of electrons in the outermost shells of an atom influences its reactivity. These shells are not spherically symmetric, so the numbers of electrons in these shells can also have an effect on how clusters of several atoms of an element are shaped (i.e. what shapes crystals will have). The shells also have energy hierarchies – traveling from one to another requires the electron to absorb a precise amount of energy from an external source, and traveling back will give off that same amount; each electron configuration (again, which is pretty much 1:1 with the number of protons) will have a different group of these energy transitions, which we can see as light. The list goes on and on.
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)
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.
Consider a Lego brick; just by changing the number of pegs you can pretty drastically change the number of different ways that it can connect to other bricks.
Chemistry works similarly; the number of protons dictates the number of electrons each atom has, which in turn affects the number of connections that specific kind of atom tends to make.
So, with say Water (H2O) you’ve got a couple 1-peg bricks of Hydrogen attached to a single 2-peg brick of Oxygen.
You rarely encounter atoms by themselves. Instead you find atoms clumped with other atoms. So how the atoms behave in groups is far more important than how they behave alone.
Electrons like to travel in pairs. This makes atoms with an odd number of electrons want to cooperate with other atoms that have an odd number of electrons so that together they have an even number of electrons.
Electrons act as very complicated waves surrounding the atom. These complex waves have preferred configurations. If an atom has more atoms or fewer atoms than needed for one of the preferred configurations, it will work with other atoms to fix the problem.
There are two basic ways for atoms to cooperate. They can share electrons, or they can donate electrons to each other. Sharing tends to bind the atoms more tightly to each other.
The shape of the waves mentioned earlier is important. For example water, which has two hydrogens and one oxygen, is bent at an angle due to the shapes of the electron waves. Carbon dioxide on the other hand, which has two oxygens and one carbon, is a straight line. This has enormous implications for the two different chemicals.
There are many ways you can measure physical properties, and the periodic table of elements helps us organize our thinking about them.
There is a [good higher level FAQ here – The periodic table and the physics that drives it](https://www.nature.com/articles/s41570-020-0195-y) (or get the PDF [here](https://annas-archive.org/search?index=&q=The+periodic+table+and+the+physics+that+drives+it&content=journal_article&sort=)
Basically we can break it down into physical properties (solid, liquid, gas, melting point etc) and chemical properties (what does it react with, and how).
On the far right of the table are a group called the noble (non reactive) gasses.
They have similar non reactive chemistry because all their electrons are in a stable state without either “extra electrons” looking to react with something else, or an “electron deficiency” looking for extra electrons to react with it.
They have similarities in physical state (gas at room temp and 1 ATM pressure)
They get heavier as you go from He to Rn.
If you look at the first column (aka Group 1) you get things that are highly reactive. Except for H, they are all solids.
The atomic and subatomic physics that leads to this regular patter was unknown in the 1800s when Dmitri Ivanovich Mendeleev put a small periodic table together.
in 1911 Ernest Rutherford’s discovery of the atomic nucleus gave us a theoretical reason for why this happens.
The old explanations mostly used concepts like orbitals, but modern physics makes it more complex.
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