To determine the electron domain of CH2O, we need to consider the number of electron pairs around the central atom, carbon (C). In CH2O, carbon is bonded to two hydrogen atoms (H) and one oxygen atom (O). Each single bond between carbon and hydrogen represents one electron pair, and the double bond between carbon and oxygen represents two electron pairs. Therefore, carbon has four electron pairs around it.

When there are four electron pairs around a central atom, the electron domain is tetrahedral. This means that the four electron pairs arrange themselves in a tetrahedral shape, with the central atom at the center and the electron pairs directed towards the corners of a tetrahedron.