Return to Blog Homepage

PCAT Chemical Processes – Oxygen

by Sam
Jan 23, 2018
PCAT Question of the Day

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^–.

Submit a Comment

Search the Blog

Free Consultation

Interested in our Online MCAT Course or one of our One-on-One MCAT Tutoring packages? Set up a free consultation with one of our experienced Academic Managers!

Schedule Now

Free MCAT Practice Account

Need great MCAT practice?

Get the most representative MCAT practice possible when you sign up for our free MCAT Account, which includes a half-length diagnostic exam and one of our full-length MCAT practice exams.

Learn More