Atomic mass molecular masses by comparing the vapor density of a collection of gases with molecules containing one or more of the chemical element in question. In the 20th century, until the 1960s chemists and physicists used two different atomic-mass scales. The chemists used a "atomic mass unit" (amu) scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to only the atomic mass of the most common oxygen isotope (O, containing eight protons and eight neutrons). However, because oxygen-17 and oxygen-18 are also present in natural oxygen this led
Avoided crossing one set of parameters. If both of the atoms of a diatomic molecule is same, the symmetry suggests that different configurations keeping their mutual distance fixed will result into same electronic states. So it is the relative distance formula_73 which acts as a parameter for the two equations promising level crossing. Hence due to the avoided crossing theorem in general we can not have level crossings between two electronic states of same symmetry. But in polyatomic molecules the number of independent mutual distances of nuclei are more. For a N-atomic molecule the number of independent mutual separation is formula_74 (for