I know that this might look a very dumb question but I have been spent my days to think about it and couldn't understand and finally decided to ask.
Consider Sharing of electrons to form covalent or coordinate bonds , elements can complete their octet by the method of sharing and unlike ionic bonds, they don't have to lose their electrons i.e. they only share and do do not lose electrons . The shared electron ( or a pair) is counted on both atoms then. For example, in Cl2 molecule, even though each chlorine atom had 7 electrons before, but after sharing, they both have 8 electrons and thus complete their octet. This is only possible if we count the shared electron in both the atoms while checking their octets.
NH3 + H(+) = NH4(+)
Consider an ammonium ( denoted as NH4+ ) , after bond formation, within the molecule, let's count for doublet of hydrogens. We find that each hydrogen had a complete doublet. Let's count for nitrogen, again, we see that nitrogen had 8 atoms and hence an octet ( the shared electron pair is also counted while counting for octet, as we do in chlorine molecule) . Now, my question is that if each hydrogen and nitrogen are having their valence shells filled, then from where an extra positive charge comes and people call it a cation .Positive charge comes when there is a deficiency of electrons , but in ammonium ; no atom is positively charged and hence the molecule.It would have been a cation if the nitrogen was positively charged, but clearly nitrogen has not lost it electrons, it had only shared . So why is there a positive charge on the ammonium?