Calculating Atomic Mass

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Presentation transcript:

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 16O, 17O, 18O

Naturally occurring isotope The relative abundance of each isotope of an element determines its atomic mass. Naturally occurring isotope Fractional abundance Mass (amu) 11C 11.011430 12C 0.989 12.000000 13C 0.011 13.003355 14C 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 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