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    Why in the nucleophilic substitution of ammonia does the nitrogen bonded to 3 hydrogen and 1 carbon have a positive charge?

    Great question! (the thought really never crossed my mind!)

    I'll try and keep my response as short as possible (unlike literally every other post I've made).

    The nitrogen donates a lone pair to the delta positive carbon. This is what a nucleophilic attack is (nucleophile donating a lone pair of electrons to a delta positive carbon to form a dative covalent bond).
    Now lets analyse the state of nitrogen before and after the bond has formed:

    Nitrogen has 5 electrons in its outer 'shell' - 3 participating in a covalent bond with 3 hydrogens and a lone pair of electrons.

    Nitrogen has still got 5 electrons, but when it forms the dative covalent bond with carbon, the lone pair it 'donates' are now in the middle of the C-N bond. Compare that to when the two electrons just 'belonged' to nitrogen. Now they're in between nitrogen and carbon, in effect, its just as if nitrogen has only got 1 of those two electrons for itself, since they are both somewhere in between the nitrogen and carbon.
    Therefore since the two electrons now only spend half the time with nitrogen, its like having 2 1/2 electrons, hence from two electrons originally it now has 2(1/2)=1 electron. Since its gone from having 5 electrons to '4' electrons (technically still 5 but) it hence will have a formal positive charge of +1.

    Sorry for my weird explanation (perhaps I'm just making this all up!). I hope I helped a tiny bit.
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