Why is the atomic mass of oxygen 15.9994?  

Expert Answers
enotechris eNotes educator| Certified Educator

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.

neela | Student

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.

revolution | Student

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)

= 16

One atom of oxygen is 16 times heavier than 1/12 of an atom of carbon-12, so the relative atomic mass is 12.

usmchem1 | Student

The atomic mass of oxygen is 15.9994 because that is the average mass of all of it's isotopes.