More Complex Molecules

This system allows us to predict the number of bonds that will be present in various molecules. For instance, oxygen gas has the formula O₂. Each oxygen atom starts with 6 electrons, 2 short of an octet. So, we can predict that O₂ will form a double bond, as below:

We can also make some educated guesses about how atoms will arrange themselves in more complex molecules. Knowing the hydrogen is stable with only 2 electrons (one bond) we can be sure that hydrogen will be on the outside of any molecule. 

In addition, carbon, which has 4 valence electrons needs to make 4 bonds to be stable and will therefore generally be found in the middle of a molecule, as the example C₂H₆ illustrates.

It is also worth noting that carbon "loves" to bond to carbon, so structures with chains of carbons are very common in the world (and in chemistry classes).

When we draw the LDS for an ion, we simply add or take away the correct number of electrons and then bond to achieve octets. Then the entire ion is placed in square brackets with the charge outside in the upper right corner. In the LDS for the cyanide ion below, the extra electron is colored in the LDS on the left, although there is no real difference between any of the electrons once the structure is created.

In general, we can say the following:

• In LDS's, each atom should end up with 8 electrons (either attached or in bonds) except hydrogen, which gets 2.
• The number of unpaired electrons indicates the number of bonds that the atom will make
• Hydrogen atoms and other atoms that only want to make one bond will be on the outside of molecules
• Carbon and other atoms that want to make lots of bonds will be in the center of molecules
• Multiple bonds can be used to help atoms reach an octet

Your Turn:

With those basic ideas in mind, you should be able to draw LDS's for the following:
ammonia (NH₃)
ammonium (NH₄⁺¹)
H₂S
Cl₂O
N₂
H₂
HNO
H₂O₂
C₃H₈
C₃H₆
C₂H₂
C₂H₅OH
CH₃Br
OF^-1
CO₂
SO₄^-2

The answers are here.
Even More Complex molecules