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1.5 Identifying Formal Charges
We often include formal charges in our drawings.
If you don't draw a formal charge when you are supposed to, your drawing will be incomplete.
You have to learn how to identify and draw formal charges.
You will have a hard time passing this course if you can't draw resonance structures.
A formal charge is a charge that doesn't exhibit the expected number of electrons.
The number can be found by looking at the periodic table, which shows the number of expected valence electrons.
Carbon has four electrons in Column 4A.
The number of valence electrons is what a carbon atom is supposed to have.
An example can be seen.
The bonds represent two electrons being shared between two atoms.
Four electrons are present.
The number of electrons is what the atom has.
The carbon atom has no formal charge since the numbers are the same.
This will be the case for most of the atoms you draw in this course.
There will be a difference between the number of electrons the atom is supposed to have and the number of electrons the atom actually has.
There will be a formal charge in those cases.
An example of an atom with a formal charge.
Oxygen is in Column 6A of the periodic table.
The C-O bond has three lone pairs of oxygen.
There are two electrons that are not being used to form a bond.
Lone pairs are drawn as two dots on an atom, and the oxygen above has three of them.
Each pair must be counted as two electrons.
The oxygen atom has seven electrons, which is one more electron than it is supposed to have.
Nitrogen is located in Column 5A of the periodic table.
It has one less electron than it is supposed to have.
If the oxygen or nitrogen atom has a formal charge, it is determined by the structures below.
Draw the charge if there is a charge.
The most important atom is carbon.
Carbon always has four bonds.
It is assumed that we know how to count to four and can figure out how many hydrogen atoms there are, so we can ignore the hydrogen atoms when drawing bond-line structures.
We were only talking about carbon atoms without formal charges.
Now that we know what a formal charge is, let's look at what happens when carbon has one.
We can't assume that carbon will still have four bonds if it bears a formal charge.
It will only have three.
We will move on to C- after first considering C+.
It can only form three bonds because it has three electrons.
The reason it has a negative formal charge is because it has more electron than it is supposed to have.
It has five electrons.
We can't use each of the five electrons to form a bond because we have the lone pair.
There are regions of space called orbitals.
The bonds can be formed by the overlap of the orbitals from other atoms or the lone pair of electrons.
Carbon doesn't have enough orbitals in its shell to form five bonds.
If you look at the drawing above, you will see that a carbon atom with a negative charge will have a lone pair and three bonds.
A carbon with a negative charge can form only three bonds, just like a carbon with a positive charge.
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