Lewis Structures: Quiz

Lewis Structures: Quiz

7 questions

1. Question 1EasyHow many valence electrons are used in the Lewis structure of $\ce{H2O}$?

   A6B8C10D3
   Show answer

   Answer: 8

   2 × 1 (H) + 6 (O) = 8: two bonding pairs and two lone pairs.

2. Question 2EasyHow many lone pairs are on the nitrogen atom in ammonia, $\ce{NH3}$?

   A0B1C2D3
   Show answer

   Answer: 1

   8 valence electrons: three N–H bonds use 6, leaving one lone pair on N.

3. Question 3MediumWhich bond is found in the Lewis structure of $\ce{CO2}$?

   ATwo C–O single bondsBTwo C=O double bondsCOne C–O single bond and one C=O double bondDAn ionic bond
   Show answer

   Answer: Two C=O double bonds

   With 16 electrons, O=C=O gives every atom an octet and all formal charges are zero.

4. Question 4MediumHow many valence electrons are in the nitrate ion, $\ce{NO3-}$?

   A23B24C22D18
   Show answer

   Answer: 24

   5 (N) + 3 × 6 (O) + 1 for the negative charge = 24.

5. Question 5MediumIn O=C=O, what is the formal charge on carbon?

   A0B+1C−1D+2
   Show answer

   Answer: 0

   Carbon: 4 valence − 0 lone-pair electrons − ½ × 8 bonding electrons = 0.

6. Question 6HardWhy are both O–O bonds in ozone, $\ce{O3}$, the same length?

   AThey are both single bondsBThey are both double bondsCThe molecule is a resonance hybrid of two structuresDThe molecule flips quickly between two structures
   Show answer

   Answer: The molecule is a resonance hybrid of two structures

   The real molecule is an average of O=O–O and O–O=O, so each bond is between a single and a double bond.

7. Question 7MediumWhich molecule has a central atom with fewer than 8 electrons?

   A$\ce{CH4}$B$\ce{BF3}$C$\ce{NH3}$D$\ce{CCl4}$
   Show answer

   Answer: $\ce{BF3}$

   Boron forms three single bonds and has no lone pairs: only 6 electrons around it.