Formal charge and net charge

Question

What is the basic difference between formal charge and net charge . Does formal charge take part in resonance .

I searched it on wikipedia , but it is mostly discussing how to find formal charge.

https://en.m.wikipedia.org/wiki/Formal_charge

Answer 1

Net charge is the sum of all formal charges of the atoms in a molecule. Net charge is the charge of the molecule.

Formal charge is the charge of an atom in a molecule. Formal charge varies when you look at resonance structure. See this post of the nitrate resonance structures. It can be obtained through:

\begin{equation} Formal\ charge\ =\ Valence\ electrons\ -\ no\ bonding\ electrons\ -\ \frac{bonding\ electrons}{2}\quad\quad\quad\quad\quad\text{(Eq. 1)} \end{equation}

Example: I take the example given previously with $\ce{NO3^{-}}$, in order to give a complete answer to the present post.

So, Having in mind the rules to draw the Lewis Structure:

• Operations

  1. Sum the valence electrons of all atoms. When the system is an anion, an electron is added. When the system is a cation, an electron is subtracted.
  2. Sum the 'octet-electron duet' of all atoms.
  3. Subtract step 2 of 1. Results corresponds to shared or bonding electrons.
  4. Subtract step 3 of 1. Result corresponds to the nonbonding electrons.

• Indications

  1. Assigns two electrons to each bond.
  2. If electrons remain from step 3. Add them in pairs creating double or triple bonds. Having in mind the duplet/octet rule.
  3. The obtained electrons from step 4. Add them in pairs to the more electronegative atoms.
  4. Calculate the formal charge of each atom.

Then:

• Operations

  1. $$valence\ electrons\ \cdot\ number\ atoms\ of\ \ce{N}\ +\ valence\ electrons\ \cdot\ number\ of\ \ce{O}\ +\ anion\ condition $$ Then $$5\cdot 1+6\cdot 3+1=24$$
  2. $$octet\ of\ \ce{N}\ \cdot\ number\ atoms\ of\ \ce{N}\ +\ octet\ of\ \ce{O}\ \ \cdot\ number\ of\ \ce{O} $$ Then $$8\cdot 1+8\cdot 3=32$$
  3. $$32-24=8$$

  4. $$24-8=16$$

     • Indications

After doing the first three steps, you will get:

(Source: Stackexchange)

Now, taken equation 1 to nitrogen:

$$Formal\ charge\ of\ \ce{N} =\ 5\ -\ 0\ -\ \frac{8}{2}\ =\ +1$$

Now, taken equation 1 to oxygen bonded to nitrogen through single bond:

$$Formal\ charge\ of\ \ce{O} =\ 6\ -\ 6\ -\ \frac{2}{2}\ =\ -1$$

Now, taken equation 1 to oxygen bonded to nitrogen through double bond:

$$Formal\ charge\ of\ \ce{O} =\ 6\ -\ 4\ -\ \frac{4}{2}\ =\ 0$$

Therefore

Net charge of $\ce{NO3^-}$ is the sum of all formal charge of the atoms in the molecule equals to -1