Stumbled across this interesting practice problem in my organic textbook. I have a few reasons for why $\ce{H4N+}$ is less acidic than $\ce{H3O+}$.

1) Let’s assess conjugate stability first. Conjugates are $\ce{H3N}$ and $\ce{H2O}$. Let’s look at central atom oxidation state. Nitrogen in ammonia has an oxidation state of negative 3. Oxygen in water has an oxidation state of negative 2. The more negative oxidation state of nitrogen suggests it is more nucleophilic than oxygen (i.e. more reactive). The more stable the conjugate base, the stronger the acid. Water appears to be more stable through oxidation state analysis.

2) Oxygen is more electronegative than nitrogen, so there is more electron withdrawal from the hydrogens in the hydronium ion. This gives the hydrogens in hydronium ion a higher partial positive charge, making them more electrophilic (and hence more reactive). This is the reason (the sole reason) my book offered.

3) Size plays a role in the direct acidities of the two molecules. First, nitrogen is bigger than oxygen. Both hydronium and ammonium ion have a positive 1 formal charge. The bigger size of nitrogen decreases its positive charge density, making it less Coulombically favorable for ammonium ion to kick off a proton.

Are these reasons 1) valid and 2) are there any more reasons I may have overlooked?

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    $\begingroup$ related: chemistry.stackexchange.com/questions/19692/… $\endgroup$ – Mithoron Apr 17 '15 at 14:08
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    $\begingroup$ Your reasoning in 1) seems dubious. I have never heard of using negative oxidation states to justify reactivity. Do you have a reference? I would be interested to learn something new. $\endgroup$ – Tan Yong Boon Oct 23 '17 at 15:26

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