The atomic mass of oxygen is often approximated to 16, but its actual atomic mass is 15.999. This slight deviation from the whole number is due to the presence of different isotopes of oxygen.
Oxygen has three naturally occurring isotopes: oxygen-16 (99.757%), oxygen-17 (0.038%), and oxygen-18 (0.205%). The atomic masses of these isotopes are:
- Oxygen-16: 15.994915 amu
- Oxygen-17: 16.999132 amu
- Oxygen-18: 17.999161 amu
The atomic mass of an element is a weighted average of the atomic masses of its isotopes, taking into account their natural abundances. Therefore, the atomic mass of oxygen is calculated by multiplying the mass of each isotope by its abundance and summing them up.
Using the isotopic masses and abundances, the calculation for the atomic mass of oxygen is as follows:
Atomic mass of oxygen = (15.994915 amu × 0.99757) + (16.999132 amu × 0.00038) + (17.999161 amu × 0.00205)
This results in an atomic mass of approximately 15.999 amu for oxygen. While this value is typically rounded to 16 for simplicity, the more precise atomic mass is indeed 15.999 amu.