The use of g/mol for diatomic molecules is due to the concept of molar mass. Molar mass is defined as the mass of one mole of a substance. While the term "atomic mass" refers to the mass of a single atom, molar mass represents the mass of a larger quantity of a substance.
For diatomic molecules, such as oxygen (O2) or nitrogen (N2), the molar mass is indeed the mass of one mole of the molecule, not the individual atoms. This is because diatomic molecules consist of two atoms bonded together, and when we refer to the molar mass of a diatomic molecule, we consider the combined mass of both atoms.
The molar mass of a diatomic molecule is calculated by summing the atomic masses of the constituent atoms. For example, the molar mass of oxygen (O2) is approximately 32 g/mol, which corresponds to the combined mass of two oxygen atoms (each with an atomic mass of 16 u).
So, g/mol is used to express the molar mass of diatomic molecules because it represents the mass of one mole of the molecule, taking into account the combined mass of the constituent atoms.