Week 7 Lewis Prelab
The answers to these questions are in the videos posted
: http://www.youtube.com/watch?v=1ZlnzyHahvo
: http://www.youtube.com/watch?v=OHFGXfWB_r4
: http://www.youtube.com/watch?v=vOFAPlq4y_k
1. How many valance electrons does the molecule HCN have?
2. If a charge of +2 were present for HCN+2, how many valance electrons would it have?
3. Write the central atom and bond polarity (polar/nonpolar) for each of the following compounds
Molecule
Ex: H2O
Central Atom
Bond
Bond Polarity
Oxygen
O-H
Polar
a. NH3
N-H
b. CO23-
C-O
c. PF5
P-F
4. Can hydrogen be a central atom for a Lewis structure?
B
5. Calculate the formal charge for each of the three oxygens and nitrogen.
A
a.
C
b.
c.
d.
6. Using the steps from the next page, complete the Lewis Structure of CCl4. If a step is not needed,
write “No” in the box.
1.
2.
3.
4.
5.
6.
Steps for Writing Lewis Structures
1. Find the total valence electrons for the molecule. Explain How
2. Put the least electronegative atom in the center and arrange the remaining atoms around the
central atom. Note: H always goes outside.
3. Put two electrons between the central atom and each of the outer atoms to form a single
chemical bond. Subtract the electrons used from the total valance electrons.
4. Complete octets on outside atoms. Note: H only needs two valence electrons. Subtract the
electrons used to complete the octet from the electrons remaining in step 3.
5. If central atom does not have an octet, move electrons from outer atoms to form double or
triple bonds. Remember that the outer atoms must have an octet when you are finished as well.
Advanced Steps
6. If you have, extra electrons after the above steps add them to the central atom.
Note: elements in the Period Three (Usually S, P, or Xe) can have more than eight valence
electrons.
7. Check the Formal Charges to make sure you have the best Lewis Structure. Explain How.
Examples: SO42- , N2O
Name: __________________
Roster #: _________
Day/Time of Lab: _____________________
Week 7 Lewis Prelab Answer Key
1. ___________ valence electrons
2. ___________ valence electrons
3. Central Atom and Bond Polarity
Molecule
Central Atom
Bond
a. NH3
N-H
b. CO23-
C-O
c. PF5
P-F
Bond Polarity
4. ____________
5.
Show all work
a. _________________
b. _________________
Work:
6.
CCl4
1.
2.
3.
4.
5.
6.
c. _________________
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