Formaldehyde, also known as H2CO, has trigonal planar geometry. This molecule consists of two single-bonded hydrogens attached to a carbon center that also has an oxygen double bond attached to it. The three atoms can only form three points around the carbon.
Formaldehyde forms chains with relative ease. In addition, it can form a cyclical compound called 1,3,5-trioxane, which is essentially anhydrous formaldehyde; it doesn't contain water. This compound is readily turned back into formaldehyde in an acidic solution. Other forms of formaldehyde found in the lab include formalin, a water-formaldehyde solution of approximately 40 percent formaldehyde with small amounts of methanol to reduce polymerization.