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In lecture, Dr. Lavelle drew out the Lewis structures for NO3- when explaining resonance. Nitrogen has 2 single bonds to Oxygen and one double bond to Oxygen, but I thought Nitrogen could only make 3 bonds because it has 5 valence electrons. Can someone please explain how this works? Can we just add however many bonds we need to any atom?
In NO3- the total number of valence electrons is 24 (N has 5 valence electrons, 3 O have 18 electrons, plus 1 more electron due to the ion's charge). When doing Lewis dot structures, the atom with the lowest ionization energy is placed in the center (in this case N) and the total number of valence electrons are arranged around and covalent bonds are created so that each atom fulfills the octet rule. Usually as many bonds as needed can be formed as long as each atom fulfills the octet rule (except for the few exceptions said in lecture).
Nitrogen has 5 valence electrons, so it needs three more electrons to fulfill its octet. This doesn't necessarily mean that it's going to make 3 bonds, but rather it starts with 5 electrons when determining the lewis structure. It will make its bonds to fill its octet.
The bonds don't just have the electrons added to get an octet, they have all of the electrons, including the valence electrons N already has. It's not limited to 3 to add on to the 5 valence electrons, you have to have the full octet in the bonds including the original 5. With two electrons being shared in each bond, and four bonds, you get 8 electrons.
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