Question

Formaldehyde is an organic compound. Each molecule includes two hydrogen (H) atoms, one oxygen (O) atom, and one carbon (C) atom. Below are the dot structures for each of these elements.

At left, a purple circle labeled H surrounded by two concentric circles, the inner with nothing on it, the outer with 1 small green spheres. In center, a purple circle labeled O surrounded by two concentric circles, the inner with 2 small green spheres, the outer with 6 small green spheres. At right, a purple circle labeled C surrounded by two concentric circles, the inner with 2 small green spheres, the outer with 4 small green spheres.
Which diagram shows how the covalent bonds most likely form in a formaldehyde molecule?

C connected above and below to H's by 1 line each. C also connected by two lines to O, which has 2 dots each above and below.

C connected above and below to H's by 1 line each. C also connected by three lines to O, which has 2 dots each above and below.

O connected above and below to H's by 1 line each. O also connected by two lines to C, which has 2 dots above.

O connected above and below to H's by 1 line each. O also connected by two lines to C, which has 2 dots each above and below.

Answer 1

The first diagram.

Reason: C forms four bonds (two C–H single bonds and one C=O double bond) to satisfy its octet. O in that diagram has a double bond to C and two lone pairs (8 electrons total). Each H has a single bond (duet). The other diagrams give incorrect bonding (e.g., a C–O triple bond would overfill carbon; O bonded to H’s would not match CH2O).