According to modern atomic theory, the atom is the smallest particle which can take part in a chemical reaction. But during the formation of hydronium ion, $\ce{H+}$ ion reacts with $\ce{H2O}$ to form $\ce{H3O+}$ ion. Here the $\ce{H+}$ ion, which is smaller than hydrogen atom, takes part in a chemical reaction. So is that postulate invalid?

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    $\begingroup$ You could argue that electrons are the most important particles in (organic) chemistry, which are of course much smaller than the atoms they belong to. $\endgroup$
    – Jori
    Feb 25 '15 at 11:03
  • $\begingroup$ There's no such thing as free proton in chemical reactions. $\endgroup$
    – Mithoron
    Feb 25 '15 at 12:28
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    $\begingroup$ @Mithonon How many free protons reach a given square centimeter of the Earth's atmosphere every second? services.swpc.noaa.gov/images/ace-epam-p-3-day.gif What happens to these free protons? bu.edu/csp/uv/proton/proton_intro.html $\endgroup$
    – DavePhD
    Feb 25 '15 at 12:49
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    $\begingroup$ @Mithoron consider proton tunneling reactions. Definitely a free proton. $\endgroup$ Feb 25 '15 at 13:38

Protons definitely participate in chemical reactions. Free protons are generally not present in liquid water because a free proton is extremely reactive, but in the upper atmosphere or in other situations where the density of matter is low there can be free protons which participate in chemical reactions.

However, I completely agree with Jori that an electron, which is much smaller than a proton, takes part in chemical reactions. Gain or loss of one tiny electron can completely change the properties of a large metalloenzyme.

How small is an electron vs. a proton? In terms of mass, the electron is 1836 times lighter, but what about in terms of radius. According to Hans Dehelt's Nobel Lecture:

Proton radius [is] $0.86 \times 10^{-15}$m...

Today everybody “knows” the electron is an indivisible atomon, a Dirac point particle with
radius R = 0.... But is it?

And goes on to explain that it is still possible the electron could have a radius on the order of $10^{-20}$ m.

See also Limit on Electron Substructure

Also, photons, which have no mass, participate in chemical reactions.

  • $\begingroup$ Yes, my immediate thought was proton and electron transfer, but great point with photons! $\endgroup$ Feb 25 '15 at 13:37
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    $\begingroup$ It is questionable to say an electron is “much smaller than a proton”. True: the proton has an inner structure, the electron apparently doesn't – in that sense, its size is exactly zero. However, that inner structure is not relevant for chemistry; what's much more interesting as a characteristic size is the quantum-mechanical unsharpness. And that is much larger for the electron, precisely because it has a smaller mass! (And yet larger for optical photons.) $\endgroup$ Feb 25 '15 at 16:23
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    $\begingroup$ @leftaroundabout Aren't you really referring to the size of a region having a given electron probability density, rather than the size of an electron itself? $\endgroup$
    – DavePhD
    Feb 25 '15 at 17:24
  • $\begingroup$ Neutrons induce many chemical transformations. $\endgroup$
    – ron
    Feb 25 '15 at 18:05
  • $\begingroup$ Well, even neutrinos may cause reactions $\endgroup$
    – Mithoron
    Feb 26 '15 at 15:25

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