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

Dec 21, 2014

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

Here are some examples

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

$1 \cdot 4 + 2 \cdot 6 = 16 {e}^{-}$

${H}_{2} O$: 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

$2 \cdot 1 + 1 \cdot 6 = 8$ valence electroncs.

${H}_{2} S {O}_{4}$: 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

$2 \cdot 1 + 1 \cdot 6 + 4 \cdot 6 = 32 {e}^{-}$.

Here's a video showing more examples: