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I know that $\ce{H2O}$ is neutral. So how does adding one more hydrogen atom to it make it positive ion ($\ce{H3O+}$)?

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    $\begingroup$ Hi Hark, welcome to Chem.SE! Where did you read that addition of an hydrogen atom forms a hydronium ion? In fact, it doesn't. Hydronium ions are formed by addition of a proton to a water molecule. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 11:51
  • $\begingroup$ So all ions are formed this way? $\endgroup$ – Hark Feb 24 '18 at 11:53
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    $\begingroup$ Ions are always charged. You cannot hope to form them by combining one atom with another atom. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 11:54
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    $\begingroup$ Deprotonation of sulphuric acid. Twice. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 11:58
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    $\begingroup$ A "neutral ion" does not exist. Neutral gases of a similar formula - SO3 and SO2 - do exist, though they are not ions. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 12:01
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The flaw in the question is exactly what Gaurang Tandon said: $\ce{H3O^+}$ is formed by a water molecule and a proton ($\ce{H^+}$), not a hydrogen atom.

Compare the number of electrons in both sides of the following two equations:

$$\ce{H2O + H^+ -> H3O^+}$$

$$\ce{H2O + H^. -> H3O^+}$$

The last equation is wrong: there's an extra electron in the left hand side.

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