We have the total of 12 valence electrons for the H₂CO Lewis structure. starting by putting the Carbon in middle, and then the Hydrogens always go on outside, and then the Oxygen will go upwards. . We have 12 valence electrons.
We'll put 2 between atoms to form the chemical bonds and then on the outside to fill the octet of the Oxygen. So we just have the 2, 4, 6, 8, 10, and 12 electrons. At this point, everything has the full outer shell except the Carbon, which only has six valence electrons. We can take these 2 valence electrons to from Oxygen and then the share them to form the double bond. Now the Oxygen has 8 valence electrons, still has 8, but Carbon has 8. Thus ,the Hydrogens, it only need 2 for a full outer shell, so they're fine.
So in Lewis structure for H₂CO, it has completed the octets for each of the atoms, and where only 12 valence electrons are used.
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