Problem: Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?

FREE Expert Solution

Recall that the octet rule states that an element is surrounded by eight electrons in the Lewis structure. 


Exceptions to this rule occur when:

• there is an odd number of electrons, like in radicals

• there are less than 8 electrons or incomplete octet

• there are more than 8 electrons or expanded octet. This occurs to nonmetals from Period 3 to 7.


carbon dioxide, CO2

  • C (less electronegative than O) → C is the central atom


The total number of valence electrons present in CO2 is:


            Group             Valence Electrons

C         4A                   1 × 4 e4 e

O         6A                   2 × 6 e12 e

Total:  16 valence e


The Lewis structure for CO2 is:



C: 8 val. e-,  O's: 8 val. e- each 


CO2 satisfies the octet rule.


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Problem Details

Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?

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