Which of the following is NOT necessary for molecule formation?

Molecule formation primarily involves the interactions of atoms through their valence electrons, which are essential for forming chemical bonds. Chemical bonds, such as covalent and ionic bonds, are crucial for holding the atoms together within a molecule, and as such, multiple atoms are required to create a distinct molecular structure.

External energy is not an absolute requirement for all types of molecule formation. While certain processes, such as initiating reactions or overcoming energy barriers, may require external energy input (like heat or light), many molecules can form spontaneously when conditions are favorable, such as in stable environments or through simple combination reactions at ambient temperatures. Therefore, its absence does not prevent molecule formation, making it the correct choice for what is NOT strictly necessary.