An atom is composed of a positively charge nucleus, with a cloud of electron(s) around it. But I can seem to understand the huge differences between 2 components that seem so close in terms of what they are made of.

I don't understand why adding one or 2 electrons, neutrons or protons, to get an other element, completely changes the behavior of said component.


Quantum mechanics determines the possible states of electrons in an atom. Whereas a planet could theoretically orbit a star at any distance, electrons have discrete properties described by a few numbers. These states can be ordered by energy and other factors that determine which states the electrons occupy. But perhaps the most important rule, the exclusion principle, prevents any two electrons from having the same state. So instead of all adopting the "ground" state, they spread out in a pattern called an electronic configuration. Each element has one more electron than the last, resulting in an update in the configuration; and the "outermost" (top-state) electrons are those most readily available to chemical reactions.

If you want to learn more about the specifics, read about periodic law. Consequences include patterns in the sizes of atoms, how the atoms interact with light (by changing an electron's energy by the amount of energy in one photon of light), and how readily they gain or lose electrons in the manners required for engagement in specific chemical reactions.

The exclusion principle also affects how the protons and neutrons in a nucleus are arranged, which has some implications for nuclear physics. But a proton is different from a neutron and can access the same state as a neutron does. There are a few other complications unique to nuclear physics too, which you can learn about by researching the semi-empirical mass formula.

  • $\begingroup$ Thank you for your answer. I studied the electron configuration in atoms this year (however the part about the exclusion principle in the nucleus I didn't know), but the thing is how can you physically explain things like taking fluor and adding one electron, one proton and one neutron making it neon (halogen) that suddenly doesn't want to interact with any other atom. $\endgroup$ – Gornemant Dec 12 '17 at 21:58
  • $\begingroup$ @Goremant Neon has 8 outer electrons, saturating the $2s$ and $2p$ subshells against covalent bond formation, whereas fluorine has one fewer electron and can change that by forming a covalent bond that lets it share another electron originating from some other atom. $\endgroup$ – J.G. Dec 12 '17 at 22:04
  • $\begingroup$ Alright but are these just empirical observations or are they explainable using physics? $\endgroup$ – Gornemant Dec 12 '17 at 22:06
  • $\begingroup$ you don't need to add one proton, one neutron, and one electron to florine to make it neon. The element is determined by proton number only. $\endgroup$ – jmh Dec 13 '17 at 0:06
  • $\begingroup$ @Gornemant Fluorine has an "empty spot" in its top orbit. This physically means that the attraction of the fluorine nucleus extends through this "hole" outside to "steal" electrons from other elements. This is why fluorine is so chemically active. Once this "empty spot" is filled by a "stolen" electron, the orbit is complete meaning the positive nucleus is fully shielded by negative electrons like in neon, except the atom of neon is neutral while the ion of fluorine with a "stolen" electron is negative and attracts the positive ion of the element, from which the electron was "stolen". $\endgroup$ – safesphere Dec 13 '17 at 7:15

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