NH4+

[kevinav]

To form an octet, N can form 3 bonds. In NH4, however, N forms 4 bonds. How is this possible? Also if there is an additional bond, wouldn't that mean there is an extra electron in the N? If so, how does NH4 have a positive charge?

[Neil Hsu 2A]

The fourth hydrogen is attached with dative covalent bond, otherwise called a co-ordinate bond. In this bond, both the electrons come from a single atom. How the bond between the fourth hydrogen and the nitrogen is formed is that the hydrogen would be ionized to H+ (hydrogen nucleus) and it would accept two electrons from nitrogen (the lone pair), forming a dative covalent bond. NH4 has a positive charge because of the formal charges on the atoms, 0 on all the hydrogens and +1 on the nitrogen, which is where the positive charge comes from.