Why is the atomic mass of oxygen 15.9994?
Oxygen has 3 different stable isotopes. The most prevalent version of these is Oxygen 16, which has 8 protons and 8 neutrons, for a total atomic mass of 16. O16 accounts for 97.765% of all oxygen atoms on Earth. Oxygen 17 contains 8 protons and 9 neutrons, for an atomic mass of 17, and Oxygen 18 contains 8 protons and 10 neutrons, for an atomic mass of 18. If all (100%) oxygen atoms were Oxygen 16, the atomic mass for all oxygen atoms would be 16; the addition of the two rare isotopes, since they are heavier, make the average atomic mass higher.
However, average atomic mass is listed as less than 16 -- this is because there are also radioisotopes of oxygen -- meaning that there are isotopes of oxygen that are radioactive. There are 9 of these, going up to an atomic mass of 24 (which means the oxygen has 8 protons and 16 neutrons!) These, however, are relatively short lived. The preponderance of the radioisotopes O12, O13, O14, and O15, with 4 through 7 neutrons, respectively, lowers the atomic mass to be just under 16.
Atomic mass and atomic weight are different. Atomic mass refers to the mass of an individual atom of a particular isotope. The atomic mass is, therefore, the total mass of nutons, protons and also electrons(though negligible) of one single isoptope of the element.Whereas, the atomic weight is a weighted mean of ratio atomic masses of the isotopes of an element to the 1/12 of atomic mass of carbon-12, the weights being their natural abundance of the isopes in a planet or a given situation. The natural abundance may vary from planet to planet.But at a particular time in a planet the atomic weight remains constant and may vary slightly over time.
Oxygen has several isotopes. But the most stanble isotopes and their atomic masses are as below:
Isotopes Protons Nutrons Atomic mass Aundance fraction
16O 8 8 16 0.99757
17O 8 9 17 0.00038
18O 8 10 18 0.00205
Therefore, The atomic weight of oxygen is
(16*(0.99757)+17*(0.00038)+18*(0.00205)/(1/12 of atomic mass of carbon12) = 16.00448 .
Again, the atomic mass of an isotope is a whole number as there is no fractional protons or nutrons are there.
The relative atomic mass of any atom is the number of times the mass of one atom of an element is greater than 1/12 of the mass of one carbon-12 atom.
For examle, oxygen has an mass of about 8, so the answer should be
Relative atomic mass= 8/ (1/12 of 6)
One atom of oxygen is 16 times heavier than 1/12 of an atom of carbon-12, so the relative atomic mass is 12.
The atomic mass of oxygen is 15.9994 because that is the average mass of all of it's isotopes.