Why does changing just one proton in the nucleus of an atom make a whole new element? The individual protons in each of these three elements are almost exactly the same, only the number changes. How can a single proton make such a huge difference in an element’s properties?
The simple answer is: The number of protons in the nucleus determines the number of electrons the atom needs in order to be neutral. The number and configuration of the electrons of an atom determines its chemical properties. So since the number of electrons is determined by the number of protons, changing even just proton will change the chemical properties of the element.
If the above is true, then why don’t we classify elements based on their number of electrons instead of protons? The reason is because electron numbers for most atoms, can be changed by taking on or giving away electrons to and from other atoms. This is the basis of chemistry. But the change in electrons does not affect the element's essential nature, that is still retains its atomic properties.
But the number of protons essentially never changes for most elements. It remains the same because protons cannot be exchanged with other atoms like electrons can in chemical reactions. So the proton count of an element does not change in chemical reactions. This proton number, in turn, determines the number of electrons the atom needs to be neutral. And that in turn, determines the behavior of the atom when it interacts with other atoms chemically, that is, the bonds it can form. And this behavior determines both its chemical and physical properties.
So there is nothing magical about the individual proton. But a change in proton number determines how many electrons an element needs to be neutral, and the propensity of that element to keep, give away, or share its outermost electrons with other atoms.
Electrons in the outermost shell of an atom that determine its chemical properties. Why are there different electron shells?
Atoms and molecules tend to favor the state with the lowest potential energy, because of the second law of thermodynamics - the law of entropy.
Solving the Schrodinger equation shows how the energies of the electrons in any given atom will be distributed in its ground state. And when we solve it, we find that electrons are distributed in orbitals and shells around the nucleus.
An orbital can contain only a maximum two electrons due to the Pauli exclusion principle. Solving the Schrodinger equation also shows us that as the number of electrons increases in an element, they occupy different energy levels or shells around the nucleus of the atom. These shells can only accommodate a maximum of a fixed number of electrons. These numbers are 2, 10, 18, 36, 54, 86, 118. These numbers are determined by solving the Schrodinger equation.
So for the handful of elements that have exactly these protons numbers, they will have the precise number of electrons that make their atomic structure most energetically stable. Consequently, they will not have the propensity to take on or lose any of their electrons to other atoms. These are the Nobel elements.
Chemistry works by elements trading electrons to form neutrally charged systems that are more energetically favorable, than the elments on their own. This is why the proton number is key in defining the element, as it is the main factor in determining what number of electrons an element would prefer. It all boils down to making a system that is more energetically favorable and electronically neutral.
If full electrons shells around an atom is most energetically favorable, then why aren’t all elements noble elements. Why didn’t nature make only these most stable elements?
The reason is that most elements were formed in fusion reactions within the cores of stars or star processes. And the fusion process results in nuclei with all kinds of different numbers of protons, not just the noble elements because fusion is a nuclear process that just makes stable nuclei. It’s not a chemical process that optimizes electron shell stability.