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MCAT Chemistry Question — Oxygen

What are the formal charges on oxygen A, carbon, and oxygen B in this resonance structure of CO2?

mcat sample question biology

A. -1, 0, -3

B. +2, 0, +2

C. +1, 0, -1

D. +5, +4, +7

Click for Explanation

Formal charge = valence e – ( e in lone pairs + 1/2 bonded e)

Stated more loosely, the formal charge is the number of electrons an atom “wants” to have minus the dots minus the sticks. So, for example, for the oxygen on the left, we know that oxygen “wants” to have six electrons. It’s got two dots and three sticks. So 6 – 2 – 3 = +1


A. -1, 0, -3, incorrect, Uses formal charge = -valence e + ( e in lone pairs + 1/2 bonded e).

B.  2, 0, 2, incorrect, Uses formal charge = -valence e + ( e in lone pairs + bonded e).

C. 1, 0, -1, correct.

D.  5, 4, 7, incorrect, Uses formal charge = e in lone pairs + 1/2 bonded e.

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