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How can an element accept a proton? Isn't this practically impossible? An atom can take an electron and place it in its valence shell to complete its octet or duplet, but, the proton sits back in the nucleus with the neutron. So, it must be impossible for an atom to take a proton and bring it in the nucleus which sits deep in the centre.

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    $\begingroup$ Look at your equation and think: if the nitrogen atom would take the proton to its nucleus, would it still be nitrogen? Did the hydrogen atom count change? $\endgroup$
    – vapid
    Nov 28, 2016 at 7:57

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Hydrogen is nothing but a proton with an electron about it, so H+ is just a proton.

Therefore it is not the element that accepts the proton, it is the molecule.

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The Brønsted-Lowry theory is valid. When indicators are used in titration for example, the color of the solution, depends upon how many H's are bonded to the chemical indicator which is related to the acidity or basicity of the solution. Generally, the Brønsted-Lowry theory would tell you that the molecule with the most H's would donate it's proton (H) to the molecule with least H's when this happens NH4+ becomes NH3 upon losing an H, and NH2- becomes NH3, so you got two equivalents of NH3.

But that is not quite the best model as the Lewis-base model is better where the base is the electron donator (NH2-), and the acid is the electron receiver (NH4+). I might have to shift into my virtual machine to draw you a diagram. But anyway, what is happening is NH2- has two lone pairs of electrons and an negative charge, while NH4+ has no lone pairs of electrons and a positive charge ... so there is electrostatic attraction between the two molecules. A lone pair of electrons on NH2- desires to form a stable molecular orbital (as a lone pair orbital is not as stable as a molecular orbital in a bond between two atoms) so the lone pair of electrons abstracts a free H from NH4+, and as this happens a bond breaks from the H being abstracted to its N on NH4+ as H doesn't like to form two bonds, so NH4+ becomes NH3 and NH2- by gaining an H becomes NH3, as well.

Keep in mind that the bond between N-H, C-H, O-H, etc is polarized (not strong) because there is a large difference in electronegativity of the elements N and H, so it's easy to lose an H from whatever element it's bonded to if it feels a pull of free electrons.

New molecules form by the flow of electrons between atoms on one molecule and atoms on another molecule. It's possible to form monatomic ions like when you dissolve H-Cl in water and get the free chloride ion with a full octet of electrons. However, atoms within molecules usually get their full octet by bonding to other atoms from other molecules and sharing the electrons covalently.

Anyway, here is the electron-flow diagram. Those arrows show the movement of electrons. Note that Any of those H's of NH4+ is up for grabs. Electron Flow Diagram

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