What is the average atomic mass of an elements naturally occurring atom isotope?

Answer 1

What is the average mass of an element's naturally occurring atom or isotopes?

The average mass of an element's naturally occurring isotopes is the sum of the (isotope's atomic mass * % Natural abundance)

Below are the 3 naturally occurring isotopes of Oxygen and their % Natural abundance.

O-16 15.99491463 99.757%

O-17 16.9991312 0.038%)

O-18 17.9991603 0.205%

Now multiply and find the average atomic weight of Oxygen

O-16 (15.99491463 * .99757) = 15.95605

O-17 (16.9991312 * 0.00038) = 0.00646 O-18 17.9991603 * 0.00205) = 0.036898

Sum =15.95605 + 0.00646 + 0.036898 = 15.9994

My Periodic Table has the Atomic Mass of Oxygen as 15.999. Yee Haw!

Answer 2

Approximately 20 elements such as yttrium and sodium have only one stable, naturally-occuring isotope. All remaining elements have at least two isotopes. The atomic mass of any stable element as shown in the periodic table is the weighted average of all of that element's stable, naturally-occuring isotopes. Thus, except for the elements that have only one stable, naturally-occuring isotope, none of a given element's isotopes have the atomic mass shown in the periodic table; each isotope has its own atomic mass.

What may be confusing to some students is the way all isotopes are named and the fact that no stable isotope has a whole number as its atomic weight - there is either some mass "missing" or some "extra" mass. Isotopes are denoted by "nuclide symbols." Let's look at chlorine (Cl) as an example: There are two natural and stable isotopes of Cl. The nuclide symbols are 35Cl and 37Cl, the respective atomic masses are 34.969 u and 36.966 u, where u stands for atomic mass unit, and the natural abundances of the isotopes are 75.76% and 24.24%. The weighted atomic mass for Cl is (34.969u x 0.7576) + (36.966u x 0.2424) = 35.45u. This is the atomic mass shown on the periodic table (to four significant figures). Clearly, both Cl isotopes appear to be missing some mass. The difference in mass between what is indicated by the nuclide symbol and the actual mass of the isotope is due to the fact that the isotope was formed from nuclear fission in a star. The tremendous amount of energy released in a nuclear fission reaction comes from mass being converted to energy per Einstein's equation E = MC2.

Answer 3

The atomic weight sometimes called the relative atomic mass. See Wikipedia article "Relative atomic mass".

Answer 4

It would depend on which element and what the isotopes of the element are.