Calculating Atomic Mass The atomic mass of an

Calculating Atomic Mass

§ The atomic mass of an element is a weighted average mass of the atoms found in nature. § If you were to mass an oxygen atom, would it weigh 15. 9994 amu? § NO! 15. 999 amu – THIS IS ONLY AN AVERAGE OF ALL THE ISOTOPES OF OXYGEN THAT EXIST IN NATURE § 16 O, 17 O, 18 O

The relative abundance of each isotope of an element determines its atomic mass. Naturally occurring isotope Fractional abundance Mass (amu) 11 C 0 11. 011430 12 C 0. 989 12. 000000 13 C 0. 011 13. 003355 14 C 0 14. 003241

§ Three isotopes of oxygen occur in nature: oxygen-16, oxygen-17, and oxygen-18. The atomic mass of oxygen is 15. 9994 because oxygen-16 is the most abundant.

QUESTION § If element Z has two naturally occurring isotopes: Z-20 and Z-22 and the atomic mass of Z is 21. 5 amu § Which isotope occurs most often in nature? § Z-22 § the relative abundance of each isotope in nature is one Z-20 to two Z-22.

§ Thallium has two isotopes, thallium-203 and thallium-205. Thallium’s atomic number is 81 and its atomic mass is 204. 38 amu. § So, is there more Thallium-203 or Thallium 205? § There is more Thallium-205 found in nature. § Atoms of both isotopes have 81 protons. Thallium-205 atoms have more neutrons.

§ A sample of oxygen contains three naturally occurring isotopes:

The relative abundances and atomic masses are: § 99. 7759% (mass = 15. 995 amu) § 0. 037% (mass = 16. 995 amu) § 0. 204% (mass = 17. 999 amu) § Calculate the average atomic mass of the oxygen.

§ (percent x mass) + (percent x mass) and so on…. § 0. 99759(15. 99491) + § 0. 00037(16. 99913) + § 0. 00204(17. 99916) = § 15. 999 amu