C16
Note that for this explanation all electrons mentioned are those that appear on valence shells only.
You can look at it in two ways:
Carbon has 4 electrons, oxygen has 6. You have 1 carbon atom and 2 oxygen atoms.
Add them together, and you get a total of 16.
Or you can look at it this way:
Refer to the Lewis structure of CO2.
Each oxygen has 2 lone pairs.
2 oxygens 2 lone pairs per oxygen 2 electrons per pair = 8 electrons
There are 4 bonds total in the Lewis diagram: 1 double bond for each carbon to oxygen.
4 bonds * 2 electrons per bond = 8 electrons
Add those together, and you get a total of 16.
