When drawing a Lewis diagram for any molecules. You have to know that there are some molecules that violate the octet rule, i.e. atom must have 8 electrons around them. When we talk about violation. it means “that some atoms can have less than 8 valence electrons or have more than 8 valence electrons around them.” Now, let look at the exception in category.
Incomplete octet exception
Elements like carbon, nitrogen, oxygen, fluorine, i.e. element that are in period 2 on the periodic table. Looking this set of element we can see that can’t have up to 8 valence electron, i.e. they can either be 1, 2, 3, 4, 5, 6, 7. Note that in the second energy level the maximum number of electron is 8. Hence, we can say that period 2 element will have an incomplete octet. Let take a look at the following examples:
Expanded octets exception
Elements like phosphorus, silicon, sulphur, chlorine, i.e. element that are arrange in period 3 on the periodic table. This elements are said to have an expanded octet because they are having more than 8 valence electrons. Knowing that any element in period 3 or below can have an expanded octet, i.e. they all tends to have more than 8 valence electron. It might be 9, 10, 11, 12…. Let take a look at the following examples:
Molecules having an odd number of electrons
If you calculate the total number of valence electron of a molecule, and you find out that it is an odd number, i.e. number that are not divisible by 2. Then it violate the octet rule. But, If you calculate the total number of valence electron of a molecule, and you find out that it is an even number. Then it does not violate the octet rule.
Note the following while dealing with Molecules having an odd number of electrons octet violation.
- The bond that each element will like to form.
- One of the atom will have an incomplete octet. Hence, the other must have a complete octet.
- The most electronegative element must have a complete octet.
- Central atom will have an incomplete octet.
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