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There are quite a few sources (mostly high-school physics textbooks) that I've read which don't give the disclaimer that the hydrogen atom they are using in a diagram is an isotope (as in having unequal neutron and proton count).

Why does the "typical" (in quotes due to a lack of a more scientific or precise term) hydrogen atom have 1 electron, 1 proton, but no neutron?

What's the reason?

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    $\begingroup$ Taken from wiki: "Nearly all deuterium found in nature was produced in the Big Bang 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 to deuterium (about 26 atoms of deuterium per million hydrogen atoms) has its origin from that time." $\endgroup$
    – jim
    Commented May 23, 2016 at 20:27
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    $\begingroup$ Yes, at 26 deuterium atoms per million hydrogen atoms it's the most obvious thing to use. $\endgroup$
    – jim
    Commented May 23, 2016 at 20:32
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    $\begingroup$ A isotope is any variant of a element. The ones you think of as common aren't excluded. In any case, now that you're clear this is nitpicking about semantics and your erroneous understanding of "isotope", I don't see the point to this question. $\endgroup$ Commented May 23, 2016 at 20:35
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    $\begingroup$ A hydrogen with no neutrons is an isotope of hydrogen. Sometimes it is called H, sometimes protium. A hydrogen with one neutron is also an isotope of hydrogen. When required to distinguish it from other isotopes it is called deuterium, or D. A hydrogen with two neutrons is also an isotope of hydrogen. It may be called tritium, or T. Taken all together, these are all hydrogen, collectively referred to as H. Context is critical. $\endgroup$
    – Jon Custer
    Commented May 23, 2016 at 20:37
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    $\begingroup$ @SirJony - you seem very confused about what an isotope means. And, in particular, the isotope of hydrogen with one neutron is called deuterium. The hydrogen isotopes have separate names, primarily because there are fairly obvious differences in the chemical properties of compounds with the various hydrogen isotopes. $\endgroup$
    – Jon Custer
    Commented May 23, 2016 at 20:45

3 Answers 3

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The most common isotope of hydrogen has no neutrons. Other isotopes are deuterium with 1 neutron and tritium, with 2 neutrons. Since virtually all (99.98% according to wiki) naturally occurring hydrogen comes in the no neutron isotope, it seems reasonable that books show a schematic of that one when illustrating hydrogen.

As a secondary motivation, the one electron + one proton is the simplest atom you can imagine, thus it makes a good choice.

In terms of lexicon: every atom is an isotope of a certain element. Also, every element comes in different isotopes, which differ only by their number of neutrons. I presented to you three isotopes for the element hydrogen. In this case, there is an overwhelmingly "standard" case, in terms of abundance, so we feel safe calling one particular isotope with the element's name. In other cases, this choice will not so self-evident.

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  • $\begingroup$ This is what I was looking for. Thanks! I need one clarification... If "isotope" encompass the variations of neutron count, and "atom" could mean the same thing, what's the point of having an identical definition? $\endgroup$
    – Fine Man
    Commented May 23, 2016 at 20:50
  • $\begingroup$ Isotope always refers to a particular element, and I seem to understand you can think about it more as a class, rather than as an object. Atom is a more general term, which refers to a physical system rather than as a class of physical systems. You would say: "an atom of a certain isotope of H" or "an atom of H". @SirJony But note that this is really neat-picking and maybe others think about it differently. $\endgroup$
    – Andrea
    Commented May 23, 2016 at 20:57
  • $\begingroup$ Yes, I've gotten plenty of criticism for wrong terminology from the comments under my question :-). So, what term would I use for the isotope shown Mendeleev's tables (i.e. typically - but not always - an equal number of neutrons and protons)? $\endgroup$
    – Fine Man
    Commented May 23, 2016 at 21:01
  • $\begingroup$ By "Mendeleev's tables", are you referring to a particular set of tables, or just to the periodic table? In general periodic tables, the neutron number N is generally omitted. Sometimes they quote an effective atomic weight A which results from measuring the weight of a sample of that element, which generally contains various isotopes. I never came across a name for an isotope with N=Z. It is true that in most stable isotopes, N is close to Z because of nuclear physics. $\endgroup$
    – Andrea
    Commented May 23, 2016 at 21:10
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    $\begingroup$ In this universe the answer is that shortly after the big bang almost all deuterium is converted to helium. $\endgroup$
    – Thomas
    Commented Jun 6, 2016 at 2:25
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The chemical properties of an element are always determined by the atomic number, that is, the number of protons in the nucleus. All carbon atoms have six protons, all iron atoms have 26, etc. It's the atomic number which is featured prominently in the periodic table, for example.

Until the neutron was discovered in 1932, this was fine. After the neutron was found to occupy the atomic nucleus along with protons, then another name had to be coined for atoms having the same number of protons but different numbers of neutrons in the nucleus. This term was "isotope", which means "in the same place" (on the periodic table, i.e. having the same atomic number.

Some isotopes have fewer neutrons than protons, some have the same number of protons and neutrons, and some isotopes have more neutrons than protons, but as long as a collection of atoms all have the same number of protons, then all the atoms in that sample consist of different isotopes of the same element.

In all cases, the number of protons + the number of neutrons = atomic mass number of an atom. Carbon-14, for example, contains 6 protons + 8 neutrons to make the atomic mass number of 14. U-238 has 92 protons and 146 neutrons, while U-235 has 92 protons and 143 neutrons. Chemically, both uranium isotopes behave identically, but U-235 can sustain a fission chain reaction while U-238 cannot. Those three extra neutrons make a difference on the atomic scale.

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  • $\begingroup$ This isn't exactly what I was looking for. However, it was educational nonetheless. Thanks! $\endgroup$
    – Fine Man
    Commented May 24, 2016 at 17:08
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It's possible to imagine living in a different universe where most nuclei of the element with charge 1 were deuterium, and the lighter protium was the rare outlier.

However, we don't live in that universe. Most of the ordinary matter in the universe is hydrogen (75% by mass) and helium (25% by mass) which has been unprocessed since the Big Bang. Deuterium is an especially fragile nucleus: it has no excited states, but fissions into a proton and neutron when hit by a photon with energy higher than 2 MeV. Once this fission occurs the neutron has only about fifteen minutes before it decays into a proton, electron, and antineutrino.

So even if there had been a period early in the history of the Universe when most of the matter was deuterium, the deuterium would have dissociated into free protons and neutrons unless the temperature were already below 2 MeV.

(I'm not even considering tritium, which has a half-life of about 12 years.)

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  • $\begingroup$ This is almost right. BBN takes place at fairly low T (because of deuterium photo dissociations), but at these temperatures the $d+d\leftrightarrow He$ reaction strongly favors He and leaves almost no deuterium behind. $\endgroup$
    – Thomas
    Commented Jun 6, 2016 at 2:27
  • $\begingroup$ Yes, there are several things that would have to be different for us to live in a deuterium-heavy universe; I picked just one. $\endgroup$
    – rob
    Commented Jun 7, 2016 at 15:12

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