Question

Why in formaldehyde (`CH_2O`) are hydrogens attached to the Carbon atom and not oxygen?

Answer 1

Because if they were, oxygen would be positively charged and carbon would be unstably negative. Atoms don't simply bond without regard for charge and electron count. They must obey the laws of nature.

Your isomeric possibilities are:

• Naturally, the first one is the most stable. It is positively-polarized towards carbon, and thus, carbon becomes the site for nucleophilic attack, such as by `"CH"_3"CH"_2"O"^(-)`, or `"CN"^(-)`.

• The second one is unreasonable because of the electronegativities of carbon (`"2.5`) and oxygen (`"3.5`).

Oxygen is more electronegative, so there is no reason why it would favorably be positively-charged (unless it were to be protonated by a strong acid in the true structure of formaldehyde), and likewise, carbon would prefer to be the one that is partially positive (in fact, it is).

• The third structure is impossibly unreasonable for the same reasons as above. The formal charges are, of course, not minimized, when they can and should be, as in structure 1.