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
The total number of valence electrons present in CO2 is:
Group Valence Electrons
C 4A 1 × 4 e– = 4 e–
O 6A 2 × 6 e– = 12 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.
Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?
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