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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 2

Lesson 6: Resonance and formal charge

# Formal charge

The formal charge of an atom in a molecule is the charge that would reside on the atom if all of the bonding electrons were shared equally. We can calculate an atom's formal charge using the equation FC = VE - [LPE - ½(BE)], where VE = the number of valence electrons on the free atom, LPE = the number of lone pair electrons on the atom in the molecule, and BE = the number of bonding (shared) electrons around the atom in the molecule. Created by Sal Khan.

## Want to join the conversation?

• Hi thanks for the clear explanation.
But in the 'About' below the video, its says that 'FC = VE - [LPE - ½(BE)]'.
Shouldn't it be 'FC = VE - [LPE + ½(BE)]' ??
• Good catch. Yeah it should be FC = VE - [LPE + ½(BE)].
• the number of any valence electron that can be found in any element is not the same in the periodic table, because it is not there at all, or is there a formula to find it, if so please include it in the video.
(1 vote)
• A quick trick I use to find out how many valence electrons elements have is by counting, from left to right, how far right an element is in a period (or row). For example, say carbon, it's in the second period and it's the fourth element in the period counting from the left so it has four valence electrons. Or sulfur, which is in the third period, which is the sixth element from the left so it has six valence electrons. This process works for transition metal elements too. An element like iron is a transition metal from the fourth period and is the eighth element from the left so it has 8 valence electrons.

Hope that helps.
• at 3.49, how in the upper structure the oxygen has 3 bonds?
(1 vote)
• Because it can form three bonds? Oxygen isn't limited to forming just two bonds. Really the only thing oxygen is limited by is having no more than 8 valence electrons.
• What is actually going on physically between the electrons and the nuclei in the molecules when there is a resonance structure? In the example, the double bond could be either be shared between the N and the right O, or the left O, but in both structures, all the atoms have their valence shells filled so why would the double bond switch between the right O and left O?
• the electron probability cloud merges side by side for this structure, it forms a π bond, but the bond is a giant bond formed between the three atoms, we call it the delocalized π bond or the coniugated π bond, you'll catch a better understanding of this concept in the unit about bond kinds and hybrid orbitals.
• Why can't we connect the hydrogen to the nitrogen, how do I know that it should be bonded with the oxygen?
• Well first, Sal mentions that it is nitrous acid so we should know it’s structure from the name. It has a set structure. All Sal is doing with his formal charge discussion making different resonance structures, or moving electrons around to get new valid Lewis structures.

From a more fundamental view, acids generally have hydrogens attached to oxygen atoms. This is because when the hydrogen leaves, it leaves behind a lone pair on the oxygen. The resulting conjugate base will have a -1 formal charge on the oxygen. If the hydrogen were attached to the nitrogen, this -1 formal charge would be on the nitrogen instead in the hypothetical conjugate base. Oxygen is more electronegative than nitrogen so having a -1 formal charge on oxygen is more stable.

Hope that helps.
• I'm very confused by the term "lone pair electrons". If we say two lone pairs then it's going to be four electrons, but if we say two lone pair electrons do we mean just two electrons in a lone pair?
• We usually just say a lone pair (singular) or lone pairs (plural). Each single lone pair is composed of two spin-paired electrons. So don't really concern ourselves with the individual electrons, rather just the lone pair as group.

Hope that helps.
(1 vote)
• Why don't individual atoms have full shells? They are bonded, and both covalent and ionic bonds have constituent atoms with full shells.
• Individual atoms are capable of having fully filled electron shells. The simplest examples being noble gas elements like helium and neon.
(1 vote)
• Does this calculation for shared electrons also work if there are coordinate bonds involved?
(1 vote)
• Are there any simple ways to remember formal charge rules?
(1 vote)
• No. There isn't. Not everything has short cuts.
(1 vote)
• How the formal charge was discovered??
(1 vote)
• Who the heck downvoted u? ? ?
(1 vote)