1. Calculating Atomic Mass

2. 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 amu?
NO! amu – THIS IS ONLY AN AVERAGE OF ALL THE ISOTOPES OF OXYGEN THAT EXIST IN NATURE
16O, 17O, 18O

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

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

5. 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.

6. 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  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.

7. A sample of oxygen contains three naturally occurring isotopes:

8. The relative abundances and atomic masses are:
% (mass = amu)
0.037% (mass = amu)
0.204% (mass = amu)
Calculate the average atomic mass of the oxygen.

9. (percent x mass) + (percent x mass) and so on….
( ) +
( ) +
( ) =
amu