Diagram A shows the Lewis structure (LS) of CH_2O. The formal charge on each atom is zero.
To get the formal charge (FC) on the atoms, cut each bond in half, as in <em>Diagram B</em>. Each atom gets the electrons on its side of the cut.
Formal charge = valence electrons in isolated atom - electrons on bonded atom
FC = VE - BE
<em>On O: </em>
VE = 6
BE = 2 lone pairs 2 + 2 bonding electrons = 4 + 2 = 6
FC = 6 – 6 = 0.
<em>On H: </em>
VE = 1
BE = 1 bonding electron
FC = 1 – 1 = 0
<em>On C: </em>
VE = 4
BE = 1 in each single bond + 2 in the double bond = 2 + 2 = 4
FC = 4 - 4 = 0
