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2.3 The Two Commandments
An arrow that comes from a positive charge is never drawn.
The tail of an arrow has electrons.
The heads of arrows are the same as the tails.
The arrow's head shows where the electrons are going.
The head of an arrow shows where the electrons are going.
The head of an arrow must point to a place where the electrons can form a bond or form a lone pair.
Thou will not break a bond.
For second-row elements, Thou shall not exceed an octet.
Let's focus on one thing at a time.
resonance structures must have the same atoms in the same order.
Only a trained organic chemist can be expected to know when it is permissible to violate this rule.
Some instructors might violate this rule more than once.
You should know that you are seeing a very rare exception if this happens.
When drawing resonance structures, you must never break a single bond.
You can make sure that you never violate this rule.
Make sure you don't draw the tail of an arrow on a single bond when drawing resonance struc tures.
There are only four orbitals in the second row of elements.
The four orbitals can be used to form a bond or hold a lone pair.
Each bond and lone pair requires the use of one orbital.
The most bonds the second-row elements can have is four.
They can never have three bonds and two lone pairs.
The sum ofbonds andlone pairs for a second-row element cannot exceed the number four.
The central atom can't form another bond because it doesn't have a fifth orbital.
The examples above are clear, but with bond-line drawings, it can be more difficult to see the violation because we can't see the hydrogen atoms.
It is difficult to see that the left structure has an arrow on it.
We can see that the arrow above gives a carbon atom with five bonds when we count the hydrogen atoms.
The carbon atom does not have an octet.
Even though the central carbon atom has only six electrons, this drawing is still acceptable.
If we exceed an octet, we will consider the "octet rule" to be violated.
The head and the tail of a curved arrow are reflected in our two commandments, "never break a single bond, and never violate the octet rule."
A bad tail and bad head are against the first and second commandments.
We are breaking that single bond if the tail of the arrow is coming from a single bond.
We have not violated the first commandment if the tail is coming from a double bond.
We did not violate the first commandment because the tail is on a double bond.
C+ only has three bonds.
The carbon atom will get four bonds when we push the electrons as shown above.
The two commandments were not violated.
Explain why the arrows violate one of the two commandments for each of the problems below.
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