Consider the equation $\ce{H2 + H2+ -> H + H3+}$

What is the atomic structure of the $H_3^+$ ion? I understand that the H$_3^+$ ion contains 3 protons and 2 electrons. However, how many neutrons does it contain? Couldn't $H$ be any isotope of Hydrogen, so we don't necessarily know how many neutrons?

In general, will we never know the number of neutrons within a chemical equation?


2 Answers 2


Unless otherwise stated, assume the most common isotope of hydrogen, thus no neutrons, to be predominant. Of course, there could be an occasional deuterium atom mixed in, but the no-neutron, all-hydrogen-1 structure dominates.

Note that the neutron count has an impact on mass and thus on vibrational frequencies, which in turn may impact the ability of this species to dissipate excess heat and enable the formation of stars. Our existence might well depend on the spectrum being what it is specifically with (the predominant form of) three hydrogen-1 (protium) atoms.

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    $\begingroup$ I object. Unless otherwise stated, I would assume H (as well as any other atomic symbol) to mean the mixture of isotopes with natural abundances. $\endgroup$ Feb 18 at 0:32
  • $\begingroup$ @IvanNeretin I try different wording. $\endgroup$ Feb 18 at 1:03
  • $\begingroup$ Now I agree.$\;$ $\endgroup$ Feb 18 at 8:49

The # of neutrons isnt very relevant to chemical properties because the pulling and pushing is done between protons and electrons(EM force).

Since this question is about stoichiometry most likely you need to assume H to be the mixture of stable isotopes with natural abundances.

  • $\begingroup$ With hydrogen there are important effects of doubling the mass on the nucleus from protium to deuterium on molecular vibrations. As noted in my answer it may have impacted the heat dissipation required to form stars. $\endgroup$ Feb 18 at 12:02

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