Final answer:
Among the given molecules (H2O, NH3, CH4, and CO2), none have a central atom with a formal charge of +2 in their conventional structures. All central atoms (oxygen, nitrogen, and carbon) complete their octets through bonding without retaining a positive charge.
Step-by-step explanation:
The central atom with a formal charge of +2 can be determined by analyzing the number of valence electrons, bonds, and the electronegativity of the atoms involved. Starting with CO2, carbon is the central atom with four valence electrons and forms two double bonds with oxygen, which does not incur any formal charge. However, in most instances, we expect carbon to share its electrons to complete its octet without retaining a positive charge.
For H2O, NH3, and CH4, the central atoms are oxygen, nitrogen, and carbon respectively. None of these structures typically result in the central atom having a formal charge of +2 as they all have complete octets through bonding without needing to carry a positive charge. For instance, in CH4 (methane), the carbon atom is bonded to four hydrogen atoms and has no formal charge.
Given that there's no conventional structure mentioned among the choices where the central atom has a formal charge of +2, the answer seems to be none of the given options.