The atomic mass of Carbon-12 being exactly 12 and the atomic mass of Oxygen-16 being approximately 15.99 is due to the way atomic masses are defined and measured.
The atomic mass of an isotope is determined by the mass of its protons, neutrons, and electrons. The atomic mass unit (amu) is a unit of measurement used for atomic masses. It is defined as 1/12th the mass of a carbon-12 atom.
Carbon-12 is used as the reference isotope for atomic mass calculations. By definition, one carbon-12 atom is assigned a mass of exactly 12 atomic mass units. This means that the atomic mass of carbon-12 is precisely 12 amu, as it serves as the reference point for the atomic mass scale.
On the other hand, the atomic mass of Oxygen-16 is approximately 15.99 amu. This is because Oxygen-16 has a slightly larger mass than 16 atomic mass units. This discrepancy arises due to the fact that atomic masses are based on average values that take into account the relative abundance of isotopes.
Oxygen has multiple isotopes, including oxygen-16, oxygen-17, and oxygen-18, with different numbers of neutrons. The average atomic mass of oxygen is calculated by considering the masses and relative abundance of these isotopes in nature. The slight variation in the atomic mass of Oxygen-16 is due to the presence of other isotopes and their contribution to the average atomic mass calculation.
So, while Carbon-12 has an atomic mass exactly equal to 12 amu (by definition), the atomic mass of Oxygen-16 is slightly less than 16 amu due to the contribution of other oxygen isotopes in nature.