Which of the following does NOT exist as a diatomic molecule?

Diatomic molecules are molecules composed of two atoms, and typically, these will consist of two atoms of the same element. Oxygen, hydrogen, and nitrogen are all examples of diatomic molecules. They naturally occur as pairs: O2 for oxygen, H2 for hydrogen, and N2 for nitrogen, playing essential roles in chemical reactions and biological processes.

Boron, on the other hand, does not exist as a diatomic molecule under normal conditions. Instead, boron typically forms more complex structures and does not exist as a simple two-atom molecule. This distinction is crucial in understanding molecular structures and chemical bonding, particularly in relation to the properties of elemental substances. Thus, identifying boron as the answer highlights the different behaviors of elements within the periodic table regarding molecular formation.