chemistry 1

Writing Lewis Structures

Name: ________________________ Date: _________ Score: ________

The Lewis model is one of the most common models used to look at chemical bonding. This model

has tremendous predictive power when it comes to determining molecular formulas, shapes, and even

molecular properties such as polarity. To write a Lewis structure, we use a dot to represent a single

electron. When determining these structures, the following guidelines are recommended.

 Calculate the total number of electrons for the Lewis structure. Remember to calculate only the valence electrons, and do not forget to account for any charges,

particularly for polyatomic ions. If the polyatomic ion has a negative charge, add one

electron for each negative charge, but if the polyatomic ion has a positive charge,

subtract an electron for each positive charge.

 Write the correct skeletal structure for the compound. There’s actually no way to know for certain the correct skeletal structure, but we can make a good “guess”. To

do this, remember that H atoms will always need two electrons for a full shell, so they

will never be a central atom. Also, many molecules, but not all, tend to be

symmetrical. So, if a molecule contains several atoms of the same type, they are

normally arranged symmetrically around a central atom.

 Distribute the number of electrons calculated in Step 1 around the skeletal structure, giving octets for each atom except for H which requires a duet. It is

always best to first place two electrons between every pair of atoms. Subtract this

number of electrons from your total in Step 2, and distribute the remaining electrons.

 After using up all electrons, if any atom needing an octet lacks an octet, move electrons around to form double or triple bonds. Please remember this is a last

resort. Electrons are negatively charged and do not want to be near each other—like

charges repel!

Directions: For the following examples, first calculate the total number of valence electrons, and then

write the correct Lewis structure. Make sure your electrons are clearly visible in your

structure, and if there are resonance structures, please draw all structures.

1. CH4

2. CO2

3. NF3

4. F2

5. N2H4

6. CN -

7. NO3 -

8. SCl6

9. NH4 +

10. BF3