How do you calculate the number of valence electrons in a molecule?

The number of for molecules can be calculated by adding the of all the atoms that form that respective molecule.

Here are some examples

##CO_2##: Each carbon dioxide molecule is formed from 1 C atom and 2 O atoms. We know that C has 4 and that O has 6 valence electrons, which means that the number of valence electrons for 1 ##CO_2## molecule will be

##1 * 4 + 2 * 6 = 16e^-##

##H_2O##: Again, each water molecule is formed from 1 O atom and 2 H atoms. Since the number of valence electrons for O and H are 6 and 1, respectively, one molecule of water will have

##2 * 1 + 1 * 6 = 8## valence electroncs.

##H_2SO_4##: One molecule of sulfuric acid has 2 H atoms, 1 S atom, and 4 O atoms, each contributing 1, 6, and 6 valence electrons. So the number of valence electrons for 1 molecule of sulfuric acid is

##2 * 1 + 1 * 6 + 4 * 6 = 32e^-##.

Here's a video showing more examples: