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I just read this:

A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons. The nuclide concept (referring to individual nuclear species) emphasizes nuclear properties over chemical properties, whereas the isotope concept (grouping all atoms of each element) emphasizes chemical over nuclear. The neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. Even in the case of the lightest elements where the ratio of neutron number to atomic number varies the most between isotopes it usually has only a small effect, although it does matter in some circumstances (for hydrogen, the lightest element, the isotope effect is large enough to strongly affect biology).

I am trying to better understand what the base level of things are. We have this nice thing called the "periodic table of elements", and we have say "carbon" which is a nice black (graphite) or clear (diamond) thing you can hold in your hand. But then there are about a dozen "isotopes" or more. Some isotopes like hydrogen are significant enough to affect biology, while most isotopes are negligible chemically (though not nuclearly, according to this wiki page).

What I don't understand is why we are calling them "elements" as these discrete things in the first place. I don't see why every "nuclide" (I guess a more modern term than isotope) is the base thing we look at in the table: a combination of protons and neutrons, that's it. Rather than saying "Carbon12" or "Carbon13", we just say "6 proton 7 neutron thing". I'd like to know why we need the concept of "elements" in the periodic table, and if it would not be better more helpful in the long run to simply think of everything as nuclides rather than elements with isotopes.

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    $\begingroup$ The word element is just a convenient grouping that includes both nucleus and orbital electrons. Change the number of neutrons in the nucleus, and you have different isotopes of the same element. Change the number of orbital electrons, and you have different ions of the element. But as long as the number of protons is the same, we call it the same element. It is just a convenient name, or group, to call it an element with a name, so we don't have to refer to the count of anything everytime we talk about it. $\endgroup$
    – Steeven
    Jun 4, 2019 at 5:12
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    $\begingroup$ Not to forget that the chemical and visual properties depend on the number of charged nucleons. $\endgroup$
    – anna v
    Jun 4, 2019 at 7:09

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As the quote explains, the effect of extra neutrons on chemical properties is usually negligible. Protons and electrons are what matter in chemistry and biology. These fields are more relevant to real life than nuclear physics is, so the concept of an element like carbon makes sense. Nuclides are basically a minor detail. For example, carbon dioxide is a greenhouse gas regardless of whether it has Carbon-12 or Carbon-13.

The reason why neutrons aren’t important in how atoms bind into molecules is that atomic structure is primarily determined by electrostatic attractive forces between the positively charged protons in the nucleus and the negatively charged electrons, and secondarily by electrostatic repulsive forces between electrons in multi-electron atoms. Neutrons, not having any electric charge, don’t exert or feel electrostatic force.

Addendum: I thank @PM2Ring for reminding me in a comment that the mass of the neutrons do affect chemical reaction rates. I was thinking mainly about molecular structure.

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  • $\begingroup$ I would like to know more about what differences isotopes have at the chemical level, even if it is "negligible". I still don't see for CO2 why we wouldn't just say 2 6-proton particle bundles with an 8-particle bundle. Does Carbon8 or Carbon22 have an effect on CO2? $\endgroup$
    – Lance
    Jun 4, 2019 at 4:33
  • $\begingroup$ @LancePollard if you're asking how the nuclear mass affects the chemical properties that's a very different one from the question you've posted. You should post a new question specifically asking about this point. However consider whether it might be better asked on the Chemistry Stack Exchange. $\endgroup$
    – John Rennie
    Jun 4, 2019 at 4:38
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    $\begingroup$ @Lance For most people, names are easier to remember than numbers. As for the chemistry, heavier isotopes move slower, so their reaction rates are slower. $\endgroup$
    – PM 2Ring
    Jun 4, 2019 at 4:38
  • $\begingroup$ OTOH, if the isotope is radioactive, that can add complications to the chemical behaviour. A proper answer discussing the details warrants a fresh question. But highly radioactive metals tend to destroy their own structural integrity. And pure tritiated water (water made from tritium, the radioactive isotope of hydrogen) is corrosive due to self-radiolysis. See en.wikipedia.org/wiki/Tritiated_water $\endgroup$
    – PM 2Ring
    Jun 4, 2019 at 4:43
  • $\begingroup$ My guess is that there is very little difference between Carbon-8 CO2 and Carbon-22 CO2 in terms of their efficacy as greenhouse gases. $\endgroup$
    – G. Smith
    Jun 4, 2019 at 4:46

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